Memoize youtube channel ID and feed data.

Memoize youtube channel ID and feed data.

Internal youtube support and the youtube-dl extension both cause the
youtube feed URL to be fetched three times per update. Caching the feed
data from feedcore allows internal support to only load the feed once.
The lru_cache removes one of the youtube-dl fetches, not perfect, but
two is better than three. I saw a 40% decrease in update times when
using the internal youtube code.

Throwing an exception from get_channel_id_url() prevents get_cover() and
get_channel_desc() from attempting to fetch a None URL, and provides
more accurate errors.

The lru_cache on get_youtube_id() saves 1ms per youtube channel when
updating. Which adds up with a lot of channels, and might be more on
slower devices.