Lint whole JS codebase and force tab indentation

[atoppi]

As the title explains, Just fixed all the eslint errors/warnings and enforced the tab indent.

[atoppi]

The rationale for this effort is:

1. Potentially automate checks in github-actions for all the js files (currently CI is linting only janus.js)
2. Clear out the output for IDEs with native eslint support, in order not to lose any warning/error from the linter

[lminiero]

Had a first quick look at the PR. I can't say I've looked in detail at every single change (I may need to do a second review later on), but these are some things I spotted in this round.

[lminiero]

Merging.