Add a checkbox beside each user in admin user search, select all/none
links, and a button to submit all the selected users to the bulk user
actions page.  Checkboxes are reset on each new search query, but not
when changing the page or order of results.

Change-Id: If232021b232ec69dcd74d74c3d4d3c3b2248c870
Signed-off-by: Richard Mansfield <richard.mansfield@catalyst.net.nz>