Introduce two new columns to the `messages` table to handle messages being pinned.
1. `pinned` - represents a boolean `0` (unpinned) or `1` (pinned)
2. pin_time - the time the message was pinned. Necessary so we can display pins from newest to oldest.
3. 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`.