* Add a History button under the DMs where an authenticated JWT user can page
through the usernames they have stored history with, to bring up those DMs
even if their chat partner is not currently online.
* Try some fixes to the dark video detector to see if it reduces false
positives: wait for the canplaythrough video event to fire before beginning
the dark detection. Also, increase the threshold so dark frames need to be
seen 5 times in a row instead of twice before cutting the camera.
* 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.
