* For the Direct Message History database, use gorm.io as ORM so that Postgres
can be used instead of SQLite for bigger chat room instances.
* In settings.toml: the new DatabaseType field defaults to 'sqlite3' but can be
set to 'postgres' and use the credentials in the new PostgresDatabase field.
* The DirectMessage table schema is also updated to deprecate the Timestamp int
field in favor of a proper CreatedAt datetime field. Existing SQLite instances
will upgrade their table in the background, converting Timestamp to CreatedAt
and blanking out the legacy Timestamp column.
* Fix some DB queries so when paginating your DMs history username list, sorting
it by timestamp now works reliably.
* For existing SQLite instances that want to switch to Postgres, use the
scripts/sqlite2psql.py script to transfer your database over.
