R1:37b1b3163852

Maniphest - move subpriority edits to be transaction powered

Summary:
...this was nice to do for boards, since this diff also starts calling this code in the board move case. The big trick is to use the new expandTransactions hook to expand the subpriority transaction into a priority transaction if pertinent. The other stuff is just about hiding these new subpriority extractions.

...also removes the "edit" UI from the default board since we can't actually edit anything and it thus is buggy.

Ref T4422.