Implement user deletion

[range-of-motion]

As suggested by one of our users on Discord, you should be able to delete your account.

[range-of-motion]

Renamed the title to avoid potential confusion with "bank accounts".

[range-of-motion]

Sitrep.

Was thinking of just implementing a soft delete, so you can always recover your user. Then I got reminded of GPDR, which renders that solution worthless.

I propose the following.

• If you decide to delete your user there will be no recovery
• You can choose to receive your transactions in a given format (CSV or something) through e-mail
• (?) You cannot delete your user if you are the sole admin of a space with multiple users (if we would allow that, spaces could be left without anyone to control them)

[range-of-motion]

To clarify, the user record will stay intact (because otherwise we have to screw around with foreign keys), but personal data will be removed (e-mail address, name, etc.).