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 by communicated externally to the people who are supposed to get the promotion in user status, and they can then in turn redeem these vouchers. The philosophy behind this is to further reduce the amount data that can be seen by users in elevated positions.

Because of its size, this project will be split into several PRs. This is the first one which deals with the creation and destruction of vouchers.

ToDo

• [ ] Add Cypress Tests

Notes for reviewers

• [ ] 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.