refactor: Centralize server forbidden checks into moderation module

This moves all checks related to `forbidden_remote_server_names`,
`forbidden_remote_room_directory_server_names` and
`prevent_media_downloads_from` to a new `moderation` module.
This is useful for implementing more complicated logic globally.
Mostly the changes from #673, but is also relevant for #750

conduwuit-example.toml

@@ -1197,6 +1197,9 @@
 # incoming AND outgoing federation with, and block client room joins /
 # remote user invites.
 #
+# Additionally, it will hide messages from these servers for all users
+# on this server.
+#
 # This check is applied on the room ID, room alias, sender server name,
 # sender user's server name, inbound federation X-Matrix origin, and
 # outbound federation handler.