Alternate feed URLs

[strugee]

It would be useful if a podcast could have one or several canonical feed URLs associated with it, and zero or more known alternate URLs associated with it.

This would have two use cases, mainly:

• Being able to associate, for example, an MP3 RSS feed, an OGG RSS feed, and an OGG/MP3 Atom feed with a single podcast
• A foundation to build a merge queue on (so that if A and B were to be merged, B's URL could become a known alternate of A)

It would also help prevent duplicates from being submitted, because if a URL was submitted that turned out to be a known alternate URL for an existing feed, the new submission could be dropped.

I know #60 already exists but is bitrotted, so I'm not sure whether it makes sense to do this work on its own, or try to rebase that PR. /cc @stefankoegl