AMEC-2030: Refactor bootstrap migration; introduce new versioning scheme

The old numbering scheme for DB schema migrations has a number of problems,
the main one being that it forces the release version to be exactly three
single-digit numbers. This commit (which is a squash) refactors the
migration system such that:

  - The migrations that the system may perform are no longer hard-coded
    procedural logic, but are represented as objects.

  - The pre-existing, hard-coded migrations are moved from
    BootstrapMigratorServiceImpl to a new LegacyMigration enum.

  - New migrations should receive version numbers greater than 1000, which
    are independent from the release version.

  - BootstrapMigratorServiceImpl no longer inspects the POM version, but
    uses the greatest available migration number as the "binary version".
Merge remote-tracking branch 'origin/dev-AMEC-2030' into dev-corrections
Adding data migration to support correction articles