fixes #6248 / 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.
Yeah, it should be.  I must have missed this in #4226 when I changed the buttons from denied to hidden:

https://github.com/Katello/katello/pull/4226/files#diff-5d8c422d8b50b353a68d80ee6464723aL27