Archiving old migrations

After hundreds of migrations the migrations directory can get a little awkward to scroll through. It would be nice to have some way to move old migrations to a subdirectory to get them out of the way.

Start by reviewing how the migrations directory is discovered and consumed in tern. Define the intended way to move old migrations into a subdirectory, including how migration ordering and execution should behave, then verify the behavior with relevant migration scenarios.

Written by the indexing model from the issue text.