Lints new issues

[zorkow]

PR does three things:

• Lints a few issues that have been introduced during the latest PR merges
• Adds the linting rule to prefer includes over indexOf for array membership test, as discussed f2f
• Adds mandatory linting for every commit. Note,
  • this means that commit will fail if there are lint errors, but not when there are only warnings
  • it also adds a bit of extra time when committing. If this is annoying we can take it out again.

[dpvc]

Looks good.