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nipapd should refuse to start if the PostgreSQL db schema version is outdated.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