Fix#547
When selected, before the account's data is removed, some of it
is denormalized into a separate, symmetrically-encrypted table. In
particular:
- The e-mail
- All IPs used to access the account
- SHA256 fingerprints of all uploaded files
- URIs of accounts followed by or following the account
- URIs of accounts that were invited
- Some associations were missing from the clean-up
- Some attributes were not reset on suspension
- Skip federation and streaming deletes when purging a dead domain
- Move account association definitions to concern
* Add form for account deletion
* If avatar or header are gone from source, remove them
* Add option to have SuspendAccountService remove user record, add tests
* Exclude suspended accounts from search