retrieve feeds individually

In my traditional reader, I regularly access individual feeds in folders ("channels").

API-wise, this could be implementing by giving feeds a UID as well and allowing those as parameters in the `action=timeline` requests (and potentially others where it makes sense).

Start with the issue's proposal for individual feeds and review how `action=timeline` requests are defined in the Microsub specification. Determine the scope for feed UIDs and which other actions, if any, should accept them. Done means the API behavior and parameter handling are specified consistently for retrieving an individual feed.

Written by the indexing model from the issue text.