Improve accessibility of feed list and article list

Announce unread count and other circumstances for feed list items and
label the feed and article list tables.

Also label the "Filter by:" pop-up button.

Please feel free to choose different label than "Feeds" as I don't know
the proper Vienna terminology (knowing just RSS, not Vienna, I would expect
"Feeds" to be the term, but I noticed Vienna uses term "Folder" a lot in
this context). Alternatively if you tell me which term to use instead, I will
make a follow-up commit myself.