Exporting the per-package RSS/atom feed with the rest of exported APIs.

[isoos]

• Get notified by mail on package updates #1378 and Suggestion: Please add an RSS/Atom feed for each package's changelog #5267
• Moved the endpoint from /packages/<package>/feed.atom to /api/packages/<package>/feed.atom. Since it is mostly read by machines, I think it should live under the /api/ namespace, and it aligns better with the exported API files as well.
• Added extra cache purge on admin actions, as it was simpler to do than to exempt the cache use inside the per-package atom feed creation (specifically the check whether a package is visible).
• Updated and extended tests to verify the successful export.