(Bug 4532, 5142, 5143) Convert /feeds - tweaks as requested

Separated get args and post args, used automatic form_auth check,
fixed whitespace, etc.

Made one extra tweak to name.tt, removing an unneeded paragraph tag.
(Bug 4532, 5142, 5143) Convert /feeds - use default subscription filter

Found a small bug in testing where if you added a feed from the list of
checkboxes, the feed wouldn't get added to the user's default filter.
All the other paths redirect to the subscription management page.