Added a bare-bones eslint config (removing jslint)

[JohnHeitmann]

This change removes the small bit of jslint config, replacing it
with eslint. I've currently configured eslint to mostly only report
the more serious of lints, although there are still some style nits
turned on.

Eslint better supports modern js, and will be a good pre-TypeScript code
quality aid.

Install eslint with npm install -g eslint. Run with eslint html/static/*.js,
or let your IDE do it. This requires no build step.

Upcoming changes will start fixing identified bugs and other lints (mostly unused and var redef issues).

[rust-highfive]

Some changes occurred in HTML/CSS.

cc @GuillaumeGomez

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @QuietMisdreavus (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[rust-highfive]

The job x86_64-gnu-llvm-5.0 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

travis_time:end:0d3b5923:start=1543967651052981376,finish=1543967720382763039,duration=69329781663
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#Pull-Requests-and-Security-Restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
Setting environment variables from .travis.yml
$ export IMAGE=x86_64-gnu-llvm-5.0
---
    100% |████████████████████████████████| 1.4MB 920kB/s 
Collecting botocore==1.12.59 (from awscli)
/usr/local/lib/python2.7/dist-packages/pip/_vendor/requests/packages/urllib3/util/ssl_.py:122: InsecurePlatformWarning: A true SSLContext object is not available. This prevents urllib3 from configuring SSL appropriately and may cause certain SSL connections to fail. You can upgrade to a newer version of Python to solve this. For more information, see https://urllib3.readthedocs.io/en/latest/security.html#insecureplatformwarning.
  InsecurePlatformWarning
  Downloading https://files.pythonhosted.org/packages/21/cf/71dfc14692883aaf709bc1098a56770173a760a14b0b1cb74471609181be/botocore-1.12.59-py2.py3-none-any.whl (5.1MB)
    0% |                                | 10kB 33.9MB/s eta 0:00:01
    0% |▏                               | 20kB 35.3MB/s eta 0:00:01
    0% |▏                               | 30kB 40.0MB/s eta 0:00:01
    0% |▎                               | 40kB 28.6MB/s eta 0:00:01
---

[00:03:27] travis_fold:start:tidy
travis_time:start:tidy
tidy check
[00:03:28] tidy error: /checkout/src/librustdoc/html/static/.eslintrc.js: missing trailing newline
[00:03:29] some tidy checks failed
[00:03:29] 
[00:03:29] 
[00:03:29] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/tidy" "/checkout/src" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "--no-vendor" "--quiet"
[00:03:29] 
[00:03:29] 
[00:03:29] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:03:29] Build completed unsuccessfully in 0:00:56
[00:03:29] Build completed unsuccessfully in 0:00:56
[00:03:29] make: *** [tidy] Error 1
[00:03:29] Makefile:79: recipe for target 'tidy' failed
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:04c8d1ab
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Tue Dec  4 23:58:58 UTC 2018
---
travis_time:end:14b98bd0:start=1543967939131769131,finish=1543967939137031639,duration=5262508
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:06f3f64f
$ ln -s . checkout && for CORE in obj/cores/core.*; do EXE=$(echo $CORE | sed 's|obj/cores/core\.[0-9]*\.!checkout!\(.*\)|\1|;y|!|/|'); if [ -f "$EXE" ]; then printf travis_fold":start:crashlog\n\033[31;1m%s\033[0m\n" "$CORE"; gdb --batch -q -c "$CORE" "$EXE" -iex 'set auto-load off' -iex 'dir src/' -iex 'set sysroot .' -ex bt -ex q; echo travis_fold":"end:crashlog; fi; done || true
travis_fold:end:after_failure.4
travis_fold:start:after_failure.5
travis_time:start:025fa84a
travis_time:start:025fa84a
$ cat ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers || true
cat: ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers: No such file or directory
travis_fold:end:after_failure.5
travis_fold:start:after_failure.6
travis_time:start:02d73df1
$ dmesg | grep -i kill

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[GuillaumeGomez]

Thanks! For the other changes, please wait for #56005 to be merged first.

[GuillaumeGomez]

@bors: r+ rollup

[bors]

📌 Commit 5d7cf59 has been approved by GuillaumeGomez