feat(rss): Extract rss article thumbnails from enclosures

[NicolasNewman]

The current thumbnail extractor for the RSS widgets fails when its placed in an enclosure. This PR improves the coverage by:

1. If there is no media extension, check if there's 1 or more Enclosures
2. For each enclosure:
   1. If the enclosure has type image/generic, save the url
   2. If the enclosure has type "image/jpeg", "image/png", "image/gif", immediately return that URL
3. If the loop finishes without returning, return the current url (will be the first generic image, or empty string)

Comments:

• Limited by choosing the first url found. Additional filtering could be added by getting the image dimensions at the cost of fetching each resource.
• I'm not super familiar with the specification for RSS/Atom so please let me know if theres any potential flaws with this approach.
• I'm primarily using RSS feeds exported from TTRSS which moves media from the source feed into enclosures.