This introduces a versioned parent class for database migrations.

This implements a simple versioning scheme for migration helpers.

We need to be able to version helpers, so we can change their behavior
without altering the behavior of already existing migrations in
incompatible ways.

We can continue to change the behavior of helpers without bumping the
version here, *if* the change is backwards-compatible.

If not, we would typically override the helper method in a new
MigrationHelpers::V[0-9]+ class and create a new entry with a bumped
version below.

We use major version bumps to indicate significant changes and minor
version bumps to indicate backwards-compatible or otherwise minor
changes (e.g. a Rails version bump). However, this hasn't been
strictly formalized yet.

Changelog: other