[feature] Consider replies as accepted if they're in a post's "replies" collection

[tsmethurst]

Remote Accept processing was added in #3404 which helped with better distribution of approved replies across interaction-policy-aware and interaction-policy-unaware instances, but there's a piece of the puzzle still missing: fetching down a thread based on the "replies" collection.

Currently when a post with an interaction policy set on replies serves its "replies" collection, it will only include replies in the collection that have been accepted by the poster. GtS will dutifully go fetch those replies at the source, but then if the reply is from a non-interaction-policy-enabled instance, it won't have the "approvedBy" URI set, leading it to be dropped by the GtS instance doing the fetching.

To further ease compatibility between GtS and other types of instances that aren't yet interaction policy aware, it would be a good idea to mark replies in a collection as approved, as they wouldn't be there in the collection if they weren't.

Still gotta scratch my head about the best way of doing this, so this is something to play around with for after 0.17.0 is out.