Replace convoluted form building with `button_to`.
Replace convoluted form building with `button_to`.

I think historically there may have been reasons to build the forms this way
(they originally lived directly in the view, where it wouldn't have looked so
odd, and wouldn't have required `capture`), but now the same interaction can
be produced using the normal `button_to` helper.

YOTOV-LIMITED/whitehall,robinwhittleton/whitehall,ggoral/whitehall,robinwhittleton/whitehall,hotvulcan/whitehall,askl56/whitehall,ggoral/whitehall,hotvulcan/whitehall,alphagov/whitehall,askl56/whitehall,alphagov/whitehall,robinwhittleton/whitehall,YOTOV-LIMITED/whitehall,hotvulcan/whitehall,robinwhittleton/whitehall,askl56/whitehall,alphagov/whitehall,askl56/whitehall,alphagov/whitehall,YOTOV-LIMITED/whitehall,ggoral/whitehall,YOTOV-LIMITED/whitehall,ggoral/whitehall,hotvulcan/whitehall