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nipapd should refuse to start if the PostgreSQL db schema version is outdated.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
What happens if there is no comment on the database at all? Will we get a traceback here? Or perhaps on the previous line?

Somewhat nitpicking but I would prefer that you match and extract the db version rather than replace the rest of the string, e.g.
```
schema_version = None
m = re.match('NIPAP database - schema version: ([0-9]+)', comment)
if m:
    schema_version = int(m.group(1))

return schema_version
```
And when we check, we make sure we don't get a None. Also, my example includes casting the version to an integer, which means you should also store the __db_version__ as an integer and not a string. DB Version is a number, so it is just nice to handle it as such and we could potentially leverage on that to compare if the version is less than the expected (and not just different).