Use sqlite3 by default

[spapas]

Since this is a demo I'd recommend using sqlite3 (instead of postgres) for the defaultl db conn so it'd be much easier for people to quickly test it.

[leandrocp]

Linking BeaconCMS/beacon#615 for visibility. Support for sqlite3 is a WIP and it would be nice to have it on beacon_demo as well especially if we can deploy using https://fly.io/docs/rails/advanced-guides/sqlite3/ - easier to get started locally and still possible to deploy for demo purposes or for giving it a try.

[eslksw]

I second this