4840: Workbench selection actions are submitted by POST.

The previous implementation could get 414 Request URI Too Long
responses by stuffing a large number of selections into the link href.
This commit sidesteps that problem by POSTing the data with a hidden
form instead.  It's based on rails-ujs' strategy for handling links
with data-method that we were already using for some selection
actions, so it should be minimally disruptive.