show warning on "child" sources indicating they shouldn't have RSS feeds

[rahulbot]

We do allow multiple sources with the same name, iff they have a url_search_string set. Those are "child" or "placeholder" sources. We've decided a while ago that these "child" sources should not have RSS feeds one them... those should be kept on the "parent" source, the one that has the same name but no url_search_string set. I suggest adding a warning to help enforce this:

1. when you click "refetch feeds" on a child source (ie. one with a url_search_string the confirmation popup should have a ⚠️ that says "BAD IDEA - keep all RSS feeds on the parent source"
2. the "list feeds" and "create feeds" should show the same note as a banner under their header

This will hopefully help enforce the policy, but still the leave option open for the wierd-o one-offs we always end up having to for some reason or another.

[philbudne]

Just noticed this; Seems to me a corollary to this is that a rescrape of an entire collection should skip sources with a url_search_string! If so, perhaps open a separate issue for it?