Ability to Tag Subject Mappings

In preparation for allowing CQID to only process configurations that have the same tags as the subject mapping being processed, we need a way to link subject mappings to tags.

[The included test will not pass until the schema changes are applied.  In the absence of other guidance my plan will be to apply the changes once this PR is approvable, commit the sqitch tag after said deploy to this branch, and then merge the PR if the test passes with the additional commit.]