(fix): Use npm to install deps in ci.

[olizilla]

Updates Makefile to use npm@6 and the npm ci command, and updates the package-lock.json.

Local testing showed that the yarn install and build left the production bundle firing off errors, while the npm build doesn't. I don't know why. Details are in #150

Adds documentation on how the deploy happens for future code spelunkers.

Fix #150

[olizilla]

Hmm. Installing npm@6 fails in the Jenkins build. Will investigate tomorrow.

[victorb]

FYI: @magik6k added a feature to our pipeline so you can decide which branch gets deployed to which environment. Might be a good idea to have for peerpad: https://github.com/ipfs/jenkins-libs#example-with-multiple-branchesrecords

[olizilla]

@victorbjelkholm do I just need to update the Jenkinsfile to get that to work, or do I need to update anything else?

[olizilla]

@victorbjelkholm @diasdavid any objections to merging this? I don't now why a yarn build causes the errors seen in #150 but it's reproducible and switching to npm fixes it. Merging this PR will (should) get peerpad.net working again.

[victorb]

do I just need to update the Jenkinsfile to get that to work, or do I need to update anything else?

Just update ci/Jenkinsfile to have what the example I linked above has and you should be good to go!

any objections to merging this?

Nope, LGTM!