feat(rss): add image enclosure to rss feed items

[slydetector]

redlib RSS feed items don't have images so this attempts to bring it up to feature parity with teddit.

I believe this PR handles all the relevant reddit post types (image, gallery, video, gif) and even goes a step beyond for the gallery type by embedding the number of images in the description. I also made all URLs absolute as required by the RSS 2.0 spec.

Tested with /r/gifs.rss, /r/analog.rss, /r/images.rss, and a few others for about a week running on my private instance.

Also seems to fix https://github.com/redlib-org/redlib/issues/420

I'm not a rust dev, so assume I don't know what I'm doing :grin: