Add conditional requests to RSS feed reader

[charlesharries]

Description

All requests for RSS feeds are currently unconditional, which means that every time the app is loaded (and the widget cache is stale), every RSS feed is fully reloaded.

This isn't efficient, as many RSS feeds will include Last-Modified and Etag headers to indicate when a feed was last updated. If a feed hasn't been updated since last fetch, Glance should use a cached version of the feed and avoid downloading the (potentially quite heavy) full feed all over again.

I was turned onto the issue by rachelbythebay, who wrote up a decent review of how "well-behaved" feed readers should make requests:

http://rachelbythebay.com/w/2023/01/18/http/

I'm happy to put together a PR implementing these conditional requests for the RSS reader widget.