Update Headers browser test

* Create new Flow page in User_talk namespace, more likely to be in
  $wgFlowOccupyNamespaces
* Merge "I should see the edit header form" scenario into "Edit header
  on a new board" scenario
* Update selectors since edit header now reuses general edit form.
* Tweak feature annotations.

Change-Id: I90cb68faf890de572001e4c3b77b829d2faf258b