fixes #6515 / BZ 1107604 - content host package action: disable Perform on empty input

On "Hosts -> Content Hosts -> [somehost] -> Packages", disable the
"Perform" button, unless the user has provided content in the text
input.
Merge pull request #4398 from bbuckingham/issue-6515

fixes #6515 / BZ 1107604 - content host package action: disable Perform on empty input.