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nipapd should refuse to start if the PostgreSQL db schema version is outdated.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
Not sure we should give upgrade instructions in an error message.

First of all, the user is required to load functions.plsql as well, but there could also be other cautionary actions that should be performed. I think it is better left out and that the user needs to look through the upgrade instructions.