Introduce two new columns to the messages table to handle messages being pinned.
- pinned - represents a boolean 0 (unpinned) or 1 (pinned)
- pin_time - the time the message was pinned. Necessary so we can display pins from newest to oldest.
- Add an index onto pinned since we'll be querying on messages where pinned = 1 when fetching all pins.
I've also added a migration to update messages.