Display a “Success” pop up message in the middle of the form

[JessWaggs]

Users stated that they did not see the current success message in the header, or only noticed it after scanning the entire page; users said that they expected the message to appear more centrally on the screen.

YouTube Usability Report item 2.2.1.2; includes sample figure

[NedHenry]

Fixed in 37f484c