Add a way to block users

[Deuchnord]

Add configuration features to block users.
When a blocked user tries to reach the instance, the request is rejected with a 403 Forbidden response.

Note: users should be able to also block entire servers too with a * wildcard.

Examples of syntaxes:

• @[email protected]: block that user
• @*@example.com: block the entire example.com instance (should it also block all subdomains for safety?)

The blocklist is stored in the database, and is not publicly available to prevent malicious listing on public blocklists (see conversation on https://github.com/sgrigson/oliphant/issues/8).