feat(!783): Initial implementation

Adds support for extra limited-use registration tokens
stored in the database, and a new service to manage them.

src/service/mod.rs

@@ -24,6 +24,7 @@ pub mod media;
 pub mod moderation;
 pub mod presence;
 pub mod pusher;
+pub mod registration_tokens;
 pub mod resolver;
 pub mod rooms;
 pub mod sending;