Finishing touches on new Releases API

[bhearsum]

This patch adds what should be the final changes to the new Releases API for the time being. I uncovered a few bugs and needed features while working on #1290 and #1266, which this PR addresses. The changesets aren't as well organized as previous patches, but here's the overall highlights:

• Add an endpoint that can enact V2 release scheduled changes
• Add required_signoffs and product_required_signoffs to /v2/releases response
• Make it possible to cancel existing scheduled changes with PUT to /v2/releases/{name}. This can be done by posting the current version of the Release. This was needed to make it possible to edit existing scheduled changes through the UI. (Sidenote: I originally pursued a strategy of having the backend analyze the current release & scheduled change against the new proposed scheduled change, and have it create/modify/delete the release & scheduled changes as necessary -- but it proved extremely unwieldy, and I decided this was probably the better route.)
• Modify Scheduled Change response to include signoffs and when to allow the UI to update properly.
• Update rule edit endpoints to allow V2 Releases to be used in Rules
• Fix bug where sc_blob was being partly populated in /v2/releases/{name} response when only an asset had a scheduled change (ie: no scheduled change to the base)
• Ignore signoff requirements when marking a Release read only
• Fix a few cases where we treating completed scheduled changes as still active
• Always set product when scheduling changes in set_release to avoid it getting set to None.
• Fix bug where we couldn't delete Releases with scheduled changes, because the database layer complained

[nthomas-mozilla]

👍