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nipapd should refuse to start if the PostgreSQL db schema version is outdated.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
Here's the thing with schema vs db_version again. I think it is confusing.

What you are trying to show is that one is the db schema version that we should be running while the other one represents the version in the database right now, correct? I don't think it is easier to read just because one is called schema and the other one db version. Better explain it with a comment, if need be.