Improve JS (Frontend) unit testing coverage

The problem

The code coverage of the JS tests is lacking…

Unit tests: This could have been detected by JS unit tests, but those are much weaker than our PHP ones and only cover about 62% of the logic, compared to 85% (most modules are above 95%, but the cloud storage backends are badly or not at all tested, dragging the score down). In particular, the problematic line is only covered one time and the bootstrap(3) specific event handler isn't covered at all.

That was written in an aftermath of a buggy release, we think we should probably improve the code here.

The solution

• write tests, improve code coverage
  • this can be done step by step, adding small tests etc.
• Note big changes are likely easier when some refactoring like https://github.com/PrivateBin/PrivateBin/issues/365 is done

Alternatives

N/A e2e testing is related, but IMHO not strictly an alternative: https://github.com/PrivateBin/PrivateBin/issues/1335

Additional context

This issue has been discovered as a lessons learned from previous releases, which often had bugs being introduced. To have more confidence on code changes etc.

The important bit: Dear reader, if you feel like this is a thing you could contribute knowledge or actually doing it, feel free to comment here and help! This will greatly help the PrivateBin project. Also, as said above, you can start small and just contribute one or two tests and it will help the coverage to increase!