Dropdown menu for a feed

Feed dropdown menu with settings

Fixed issues: from #56
* It's not easy to click on the feed caret! Clickable area is very tiny.
* Set caret more easily clickable (still thinking it a pain to have a successful click in one try)
* Display caret only when mouse over (like Google Reader)
* Add confirmation box on "Remove Feed"
* When you add a feed with AddFeed form, new feed not show properly.
* We don't see ordering state unless you just set it. It's very confusing me.

Known issues:
* Dropdown does not show in flatui