Fix user deletion problems
Downstream MR: https://gitlab.com/soapbox-pub/soapbox/-/merge_requests/36
Deleting a remote user actually does straight up remove their user account from the database. This is a problem because no list of deleted users is kept, so the remote user is instantly redownloaded.
Instead we should handle them the same way we handle local users - by keeping the account in the database, deactivating it, and purging it. An account should only ever be fully deleted if it's local and unconfirmed/unapproved.