User: add the possibility to delete the account

[scoopandrun]

A user should be able to delete their account from the user account page.

All mentions of the user's username should then show "Account deleted" throughout the application.

It should not be possible to recover a deleted account.

A user should be able to create a new account with the same email address and username as the previously deleted one. However, this new account should not be linked in any way to the deleted one.