Membership unjoin button appears and is clickable when no joinable sites are in the list

1) Don't render the buttons if there are no unjoinable sites in the list
2) When present, disable the buttons by default; enable them only when a selection is made, and disable them again when everything is deselected
3) Change the 'Cancel' button to 'Clear Selections' (the button doesn't cancel anything, it just deselects all the checkboxes; was previously a input type=reset)
4) The message presented when you elect to unjoin multiple sites has some issues:

"You are no longer a participant in site <site1> ,<site2>, <site3>"

The spacing should be after the commas, not before, and the wording can be improved. New message will read:

"You are no longer a participant in the following site(s): <site1>, <site2>, <site3>"