Add a check for db schema version

nipapd should refuse to start if the PostgreSQL db schema version is outdated.

The up-to-date schema version (required to run) is stored inside nipap/\__init__.py
The current schema version is read out of the database comment.

The web-ui ( /version ) will also show the schema version.

Fix for https://github.com/SpriteLink/NIPAP/issues/460
I mean that it doesn't really matter if we call it schema_version or db_version (long name is probably "NIPAP database schema version"), but as long as we do it consistently.

The two variants we have here is "expected db schema version" (which nipapd expects to run properly) and "current db schema version" (the one in the database at this point). I just find it confusing that one is referred to as schema_version and the other as db_version when the real difference is "expected" vs "current in db".