Improve release workflow

[dbnicholson]

This adds a few fixes and improvements for the release workflow:

• Restore the ability to have a release created when pushing a tag.
• Make sure a release can only be run from a v* tag.
• Don't trigger app builds for major or rc releases since they'll almost certainly fail or be broken.

I ran this several times in my forked repo and am pretty sure it works as expected.

Helps: #815