Extract the state that controls mass tag form in the posts list header
to a separate class.
It's not exactly a 100% reusable control (the .tpl is shared), but it
should greatly simplify reading the JS.
- Controller lifetime is bound to route lifetime
- View lifetime is bound to controller lifetime
- Control lifetime is bound to view lifetime
- Enhanced event dispatching
- Enhanced responsiveness in some places
- Views communicate user input to controllers via new event system
