fixes #6428 / BZ 1109062 - Bulk Host Collection - disable add/remove when no hosts selected

When the user is performing a content host bulk action, if they do
not select any content hosts, disable the add/remove buttons
for host collections.
Merge pull request #4363 from bbuckingham/issue-6428

fixes #6248 / BZ 1109062 - Bulk Host Collection - disable add/remove when no hosts selected