Adds a pinned message to the rooms table

[darcys22]

The rooms table will keep track of a single i64 to track a pinned message for that room. Three associated endpoints have been added to keep track of this pinned message.

GET pinned_message Will return the pinned_message_id which should map to the pinned message to display

POST pin_message Takes a single i64 corresponding to a message id and updates the room table to keep record of this message id as a pinned message

DELETE pinned_message Resets the pinned message in the rooms table to 0 for no pinned message

All endpoints are under authorisation and POSTING and DELETING require moderator status.