Feature/account deletion

I've tried my best at implementing #115 (closed)

I did things and hope they aren't too bad, I really have no idea what I'm doing in elixir, so it is probably all terrible.

I wrote it so the user has to give their password to delete their account so it confirms their intent and that they are the owner of the account, and so it could be reused for changing passwords or something as well. I hope this is okay.

FE changes: pleroma-fe!262 (merged)