Description of problem:
Bulk select/deselect does not work properly on paginated ACS page when more than one page (the count of existing ACSs is bigger than the count per page) is available.

Version-Release number of selected component (if applicable):
6.13.0 snap 5

How reproducible:
always

Steps to Reproduce:
1. Go to Content > Alternate Content Sources page.
2. Select pagination so that existing ACSs overflow to next page(s).
3. Try to select all ACSs on the page and then deselect via the top-left checkbox.

Actual results:
Selection works, but deselection does not. All items from the dropdown (Select none (0), Select page (5)) are disabled.

Expected results:
Deselect should work too, the dropdown items should be enabled/disabled accordingly. Also, if possible, it would be great to see how many ACSs has been selected next to the checkbox, similar to what we already have in the Content View > Repositories tab.