Create invite code for teachers

[Felienne]

Currently, we manually make teachers "teacher" in the admin back end. Would be nice to be able to give people a special link with a (one time use) code that makes them teacher straight away.

[Felienne]

We already have code to mark a teacher as teacher in the Admin UI: https://github.com/Felienne/hedy/blob/7e7dbe22a68408e78d3de04eddf463e3dbed8abe/templates/admin.html#L39

So it is a matter of:

• making a new route /invite/code
• check whether the person is logged in, if so:
  • checking if the invitecode code is valid. we can do this very simple initially, just with a checksum or something for it to be ready and later move it to a DB lookup with a new table
  • calling the method above and setting the value the db
  • redirect to for-teachers
• if not logged show a message like: you need to be logged in to upgrade your account to teacher, or create an account if you have none.

[boryanagoncharenko]

After discussing the issue with @Felienne the scope of the functionality changed slightly. This issue is meant to cover the case in which the code will be printed on physical handouts which implies that it is more convenient to use a single static code. Needless to say, adding functionality for unique invitation links is still wanted: #1056

[Felienne]

Closed by #1054