ReferenceUserStatus could remember which WebFeeds they come from

[tibonihoo]

Rationale: some source (e.g. website) have several feeds, but currently wateronmars only keep track of the source(s) and not the feeds from where references have been found.

Keeping track of that could provide a better user experience in the sieve for instance and let the user know from which specific feed of a given source a reference comes.