First commit
Added the 'send more' link handling, see #6810

While the 'send more' link has been present in the layout,
it did not do anything since the handler has been missing
This commit introduces the proper handler.
Fixed error displaying mechanism to actually show an error

This is related to #6811, the freezing feedback submit button
was caused by a server error because the error handler
tried to display it using wrong function parameters. Now it should
not freeze, but instead tell the user what really happened on the
server side
Changed the form cleanup logic

I have previously missed that there was already a handler
present to make the 'send more requests' link work. The markup
has apparently been moved out of the #helpers block.
Now the logic is a little different - both the back button and the
'send more requests' link only change the blocks visibility, and the
form contents only get reset on a successful add request.
Made the email textarea get focus on certain events

Now the email input textarea gets focus on opening the
feedback helper, on cancelling the customization of the email
text and on pressing the 'send more requests' link.
Merged with the current wpcom repository state.