4291: Clean up HTTP methods in Workbench URL generators.

According to the docs at
<http://api.rubyonrails.org/files/actionview/lib/action_view/helpers/url_helper_rb.html>:

* `button_to` and `form_for` take :method as a symbol.
* `link_to` takes :method as a symbol, and only supports :delete,
  :post, :patch, and :put.  Any link that should be done with GET
  should not have a method specified.
* Note that `form_tag` *does* take a string, so not every method
  should be symbolized.
4291: Workbench uses .prop() to set element disabled property.

Per <http://api.jquery.com/prop/>: "The .prop() method should be used
to set disabled and checked instead of the .attr() method."
4291: Add Workbench controller tests for collection sharing.

I wrote these while diagnosing another problem.
4676: Collection sharing popup is always JavaScript.

This fixes an issue where the response would sometimes be sent with
Content-Type: text/html.  We thought it might be a race condition with
AJAX, but the browser was sending a correct Accept: header.
4291: Workbench Collection sharing buttons are actual buttons.

This prevents users from trying to open them in new windows/tabs and
getting a 404 response.

I had to rework the pipeline instance comparison JavaScript because it
was disabling the collection share button on page load.  All that was
really necessary was making sure the event only fires when there
actually is a form#compare, but I did some other cleanup in the
process of learning that.
4291: Workbench says explicitly when collection sharing is disabled.
Merge branch '4291-workbench-collection-sharing-button-wip'

Closes #4291, #4676, #4688.