Vouchers for user promotion - Part 1: Introduction of Vouchers

[fosterfarrell9]

In this PR, we change the way in which users can become tutors, editors or teachers. Until now, they can be selected from a dropdown. With the change implemented in this PR, vouchers are issued, which can then be communicated externally (e.g. in persona or mail) to the people who are supposed to get the promotion in user status, and they can then in turn redeem these vouchers in their user profile to get the desired position (see part 2 of this PR: #671). The philosophy behind this is to further reduce the amount data that can be seen by users in elevated positions.

This should fit in nicely into the existing workflow where mails are sent to tutors anyways for information exchange. Adding the voucher code there is not too much of an effort. Note that tutors could of course go ahead and share the voucher with other people, we cannot control that. But if somebody really does that, the teacher will notice and can intervene.

Because of its size, this feature is split into two PRs. This is the first one which deals with the creation and destruction of vouchers. The second one deals with the actual redemption part, see #671.

Out of scope

This PR also

• introduces the Cypress capability to test the clipboard content via the new Clipboard API, see Use new ClipboardAPI to replace outdated document.execCommand() calls #684
• automatically opens the dev tools in Cypress tests by default

ToDo

• Add Cypress Tests

Notes for reviewers

• In order to make the cypress tests run through, Fix lecture edit page if active term is not set #683 needs to be merged.
• Please check that generic users cannot create/destroy vouchers. This is covered in the unit tests (in vouchers_controller_spec.rb), but since this is security relevant, it would be good to check this in detail.

Preview

[codecov]

Codecov Report

Attention: Patch coverage is 84.84848% with 15 lines in your changes missing coverage. Please review.

Project coverage is 54.40%. Comparing base (78467de) to head (6046ac7).
Report is 1 commits behind head on dev.

Files with missing lines	Patch %	Lines
app/controllers/vouchers_controller.rb	68.08%	15 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #670      +/-   ##
==========================================
+ Coverage   53.86%   54.40%   +0.54%     
==========================================
  Files         157      160       +3     
  Lines        6645     6744      +99     
==========================================
+ Hits         3579     3669      +90     
- Misses       3066     3075       +9     

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[Splines]

LGTM now! I've added some more cypress tests to also test the expiration of vouchers there (via newly introduced time travelling).

Please make sure to review everything I added to this PR by means of my last commits before finally squashing and merging into the dev branch.

After the "squash and merge", I will take care of merge conflicts in the second part of the pull request.

[Splines]

the only thing left to do now is resolve the one conversation and merge dev into this branch where I fixed the flaky unit test, then rerun the tests and squash and merge