Support for database versioning

[denis-stepanov]

We need to have a version attribute of the database in the form of YYYYMMDD, which should match corresponding tags in the advent-db unit. This version should be shown on AdVent startup and with db-djv-pg dbinfo command. The exact implementation is to be decided: either a new table or a special entry in the songs table.

[denis-stepanov]

Perhaps the least intrusive option would be to create an SQL function returning a static number and override this function on updates.

The function could also return a total number of fingerprints in a given release, which would enable simple integrity checks.