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nipapd should refuse to start if the PostgreSQL db schema version is outdated.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
Well schema version is the correct term (as you are using this one too). But I think I opted for db_version in most cases as it's simply shorter.

Do you mean to say I should rename the variable to current_db_version?