Instead of the pure Go implementation of SQLite, swap in the cgo library
from mattn/go-sqlite3. On local testing this cuts the SQL query times in
half for the History modal (from 400ms per page to 150ms) and may
improve performance in production.
* Add support for storing DM history between users in a SQLite3 database.
* Opt-in by editing your settings.toml to set DirectMessageHistory/Enabled=true
* Retention days (default 90) will flush old DMs on app startup.
* On the front-end, DM history is checked when a DM thread is opened.
