Auto-remove inactive acounts

**Is your feature request related to a problem? Please describe.**

Related to #1445 - I expect many old accounts are from ex-students (or ex-employees) who don't have access to their account anymore. Also, people register accounts for courses or projects and then forget about them. Generally speaking, it's not desirable for users that their data is stored indefinitely. In the long term, storing user data indefinitely is also not sustainable.

**Describe the solution you'd like**

Periodically remove accounts that have been inactive for a certain period of time (e.g. 2 years). Send the user an email that their account will be deleted due to inactivity, and remove the account 30 days later if the user has not logged in since then.

**Describe alternatives you've considered**

#976 would allow users to delete their account themselves, which is also a good idea. However, I don't think that's a replacement for this - like I said, people can lose access or forget an account still exists.