Let apps developers edit version notes, expose it in the API (bug 945858)

- Let developers edit release notes for hosted apps
- Add id field to version API results to make it more useful, enable CORS

Also, because we don't want to make an extra HTTP query on app details pages in fireplace, the following are necessary:
- Store release notes in ES on the app
- Add release notes to app and app search API results


https://bugzilla.mozilla.org/show_bug.cgi?id=945858