Allow deleting your own account

[wil93]

We sometimes receive GDPR account deletion requests via email and we have to fulfill them manually. It would be better to have this as a self-service option.

Things to make sure:

1. Deleting the account on training also clears the corresponding account on the forum if it exists.
2. To avoid accidental deletion (or someone deleting someone else's account) it would be good to require re-entering the password and/or have a grace period of 30 days where the account can be recovered.