added version table to the db

[masukomi]

eventually we're going to have to start dealing with upgrades. like if the code is at v2 but the db is at v1 we may need to add / change columns and / or tables in the db

proposal is to create a new db_versions table with a version column that either just has one row with the most recent version it has been updated to or that has a list of all the versions it has moved through. We don't really need historical data but it wouldn't hurt and might be interesting at some point in the future.

I think the version can be an integer. I don't think there's value to using Semantic Versioning here.

should be added to hey.db.sql and default.db