Archive migrations (rather than deleting)

[CJ-Wright]

Going forward we might consider moving finished migration yaml files to an archive folder. This way if we need to reinstate them we can without too much hassle. Also it would be great to have the bot be able to re-run old migrations after the migration has finished and this seems like a good way for the bot to know what changes to make.

[jakirkham]

We can always checkout from Git history, right?

My main concern is this would grow so large as to be unusable. Though I may be overstating things.