/Codegenerated improvements

[Pettu]

We might need to break these improvements into smaller issues!

• [ ] Consider where to add the expiry time on the code. Serverside or in the DB?

• [ ] We need to tie a generated code to one specific course_schedule.

• [ ] ASDF

• [ ] EXTRA: Error validation so you can only generate one code for each class instance, or overwrite code so the teacher can "extend" the time.

[Pettu]

First off, the code from Bimi needs to be merged.