Use brave-browser-stable as the /usr/bin/ symlink for release packages

[fmarier]

This fixes brave/brave-browser#1324 by ensuring that the release Debian package can also provide an alternative (of higher priority) for the /usr/bin/brave-browser binary.

With this in place, that binary will default to the release browser and it can be configured normally using:

sudo update-alternatives --config brave-browser

As noted previously, the RPMs don't need to be fixed since they already exhibit the correct behavior.

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
  • Windows
  • macOS
  • Linux
• Verified that these changes pass automated tests (npm test brave_unit_tests && npm test brave_browser_tests && npm run test-security) on
  • Windows
  • macOS
  • Linux
• Verified that all lint errors/warnings are resolved (npm run lint)
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone

Test Plan:

I tested the following "upgrade" scenario on Ubuntu 18.04:

1. Install current stable release (0.61) and then current beta (0.62).
2. Confirm that the /usr/bin/brave-browser binary is a symlink to
   /etc/alternatives/brave-browser which in turn is a symlink to beta.
3. Confirm that update-alternatives --config brave-browser only offers beta.
4. Upgrade the stable package to the one in this PR.
5. Confirm that the /usr/bin/brave-browser-stable binary now exists.
6. Confirm that the /etc/alternatives/brave-browser symlink now points to stable and that update-alternatives --config brave-browser offers both stable and beta.
7. apt purge brave-browser and confirm that
   /etc/alternatives/brave-browser symlink now points to beta and that
   update-alternatives --config brave-browser only offers beta.

and this simpler "new install" scenario:

1. Purge all existing brave-browser* packages.
2. Install the stable package from this PR.
3. Confirm that the /etc/alternatives/brave-browser symlink points to stable and that update-alternatives --config brave-browser offers only stable.
4. Confirm that typing brave-browser launches the browser.

Finally, I built a beta version of this package and ran through this scenario:

1. Install the stable package from this PR.
2. Install the current beta (0.62) package from the official repo.
3. Confirm that the /etc/alternatives/brave-browser symlink points to stable and that update-alternatives --config brave-browser offers stable and beta.
4. Upgrade brave-browser-beta with the package from this PR.
5. Confirm that the /etc/alternatives/brave-browser symlink still points to stable and that update-alternatives --config brave-browser also offers stable and beta.

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

[fmarier]

There's no reason to prevent the stable package from providing an alternative for /usr/bin/brave-browser since it now uses a unique name for its binary (/usr/bin/brave-browser-stable).

[fmarier]

Since we no longer patch the value of ${USR_BIN_SYMLINK_NAME}, we need to make sure that the doc directory uses the package name (${PACKAGEANDCHANNEL} == 'brave-browser') and not the binary name (${USR_BIN_SYMLINK_NAME} == 'brave-browser-stable').

[fmarier]

Superseded by #2101.