Add the ability to revert Extension versions

Currently, when you upgrade an extension, you get this notice:

This has scared quite a few clients off of upgrading, because there really is NO going back. Especially for Adobe extensions, there is a strong need to be able to revert in case of issues (not to be snarky, but appMeasurement/VisitorID code updates have a history of finding major problems in the first month after release). Issues like Re: What are possible reasons why ClearVariable fails? make it clear why someone may want to move back. If you upgrade and you do see something break in staging during testing, you'd have no way to go back/compare/undo.