Enforce new ESLint rules for all .js and .js.erb files

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Enforces the JavaScript style rules introduced in #568. Note that new rules are added/customized in this PR to better reflect the reality of our codebase.

yarn run eslint --fix .

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As of a8fc972, our codebase is now fully compliant with the ESLint rules we've set up.

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This PR is now ready to be reviewed. Note that regarding testing the frontend, I've only performed some quick manual checks, e.g. made sure the login works, I can click on lectures, see a video and edit some attributes as a lecturer, e.g. add a new homework assignment.

Note that I'd love to have else and else if on the same line as the closing }. However, that's currently not possible with ESLint as far as I know. I've opened a feature request for it here. This is probably very easy to add in later on if the feature was implemented since something like this should be available as an autofix.

When reviewing you might want to use the VSCode diff view instead as GitHub does not correctly displays some changes, e.g. when we deleted tabs it shows it as deleted and added line instead of a modification.

[fosterfarrell9]

Looks good to me. I also played around a lot with things that could be effected by the refactorization of _selectize_turbolinks_fix.js, but everything seems to work nicely.