Last year, we moved to having application version codes derived automatically from the semantic version number. An application `6.6.2` would automatically get the version code `60602`. I was never entirely clear on the reasoning behind this, and it has a couple of problems:

1. New CI builds yield applications that all look to be identical versions
2. Point 1 again, because it's such an unpleasant problem.

This is now a more serious problem because we need to produce a new build that's different to the `60602` that we submitted to the Play Store, but we can't do that without incrementing the semantic version number.

I'd like to switch back to using monotonically increasing version codes. I think a reasonably sane way to implement this is to have a version-controlled file that contains a single "base" version code (such as the current `60602`), and CI builds will read the contents of this file and then add (as in arithmetically) add the current CI build number to it. We need to retain support for being able to run the CI scripts on local developer machines, so the CI build number should default to zero if it isn't present in the environment.

Acceptance criteria:

• All Firebase builds start getting unique version (build, not app version) codes again