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nipapd should refuse to start if the PostgreSQL db schema version is outdated.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
Depends on what has happened, but loading functions.plsql is never a bad idea since things are updated every now and then. Typically when we add columns, we also update trigger functions which are stored in functions.plsql. My point is mostly that I don't think we should have instructions in error messages.

The upgrade instructions are well.. uhm, someone should probably write those, hehe :P