Resolve "Add button to delete users and their content"

Closes #852 (closed)

• API to delete account
• Federate account deletion
• Remove locally known content
• Keep a track of deleted account username to avoid impersonation
• UI to delete account
• Documentation

cc @funkwhale/reviewers-python @funkwhale/reviewers-front @Sporiff

Peek_2019-09-16_17-42