Add schema version constant for CQ database

[napakalas]

The CQ database schema is being versioned in map-knowledge (see AnatomicMaps/map-knowledge#71).

The flatmap-server should define a schema version constant in competency_query.py so it can detect when the database schema is outdated and warn that an upgrade is required, rather than failing with missing table errors.

[dbrnz]

We've should allow an older competency_query.py to run against a more recent schema, as that will be the case for the staging server once the CQ database used by both staging and devel gets updated.