ESLint: enforce trailing commas and html indentation

[MetRonnie]

Enough is enough 😄

See also https://github.com/cylc/cylc-ui/issues/1536

Check List

• [x] I have read CONTRIBUTING.md and added my name as a Code Contributor.
• [x] Contains logically grouped changes (else tidy your branch by rebase).
• [x] Does not contain off-topic changes (use other PRs for other changes).
• [x] Tests, changelog and docs not needed
• [x] If this is a bug fix, PR should be raised against the relevant ?.?.x branch.