Per-package RSS/atom feed (endpoint only)

[isoos]

• Get notified by mail on package updates #1378 and Suggestion: Please add an RSS/Atom feed for each package's changelog #5267
• Introduces the /packages/<package>/feed.atom endpoint, which, at this point only lists the versions, but does not include the parsed changelog entry yet.
• Refactored the Feed and FeedEntry classes to make them more useful and have better defaults.
• Added cache purge of general and specific feeds to the generic package-cache purge entries.
• This does not include displaying the link on the package page (TBD in subsequent PR).
• This does not include the exported API sync (TBD in subsequent PR).