Redirect to beta releasenotes to fix Bug 983348

Refactor get_release_or_404 out of release_notes and
use it to determine if search for beta fallback should
occur, and redirect to public beta version if found

Also use get_release_or_404 in system_requirements so that
it now respects the is_public flag
Merge pull request #1792 from jgmize/beta-redirect

Redirect to beta releasenotes to fix Bug 983348