Update the Podcast screen UX for podcasts with the "serial" RSS tag

[mitchdowney]

@evoterra provided a list of feature improvements for listening to podcasts that have the serial RSS tag. Most podcasts with the serial tag fit the fiction, True Crime, or storytelling genres, and our default sorting of reverse chronological order is not the type of sorting and grouping that most serial podcast listeners would want by default.

I'm copy and pasting his recommendations here, and will try to keep all the work for this in one ticket.

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• Respect the >serial< - Fiction podcasts (and True Crime, docuseries, educational, and other podcasts) often are designed to be listened to from start to finish, from season 1 to whatever is next. So when the itunes:typeserial is detected in the <channel> it should signal to apps to display the episodes differently than episodic shows.
• Group episodes by <itunes:season> numbers and allow for navigation between seasons
• Display each season's episodes in the order set by the <itunes:episode number> tag in ascending order
• Put show/season trailers (<itunes:type>trailer) episodes at the top of the feed/show listing/season listing
• Move bonus episodes (<itunes:type>bonus) episodes to the bottom of the feed/show/season listing
• Use the season and episode numbers to display "S1E9" or something like that along with the <title> of each episode. (Or, if <itunes:title> exists, use that, as it's less likely to have season/episode descriptors in the title, though certainly not always)
  Those are the biggies. And please also scan for the new <podcast> namespace tags that replicate this. I use Apple's tags because they're more universal.

Nice to haves for fiction podcasters (and others):

• If <link> tags are used with the <item>, display "get more info on this episode" or something like that
• Display episode-level tags if <img> tag found in <item>s

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Related to the mobile implementation: podverse/podverse-rn#1883

[evoterra]

It me, again!

A quick comment on the prevalence of <itunes:foo> tags: I know there's a movement—which I support—to replicate those proprietary tags with <podcast:foo> tags. So whatever is done that keys off the Apple tags should also be scanning not-Apple tags, adjusting as the RSS2.0 spec changes. Please and thank you!