Keep track of the db version in a central file

[BenjaminPelletier]

Currently, the database version needs to be incremented in multiple places when a change is made. We should, at a minimum, document where all these places are. But, a better solution would be to have a single place from which everything obtains a shared piece of information (the database version) from.

Originally posted by @vosst in #349

[BenjaminPelletier]

An intermediate step could be to document all the places where the database version should be changed when it is incremented in development.

[BenjaminPelletier]

A list of places the database version needs to be updated may be found in build/deploy/db_schemas/README.md.