Werk #14755: Fix selection on "Activate pending changes" page

This werk fixes two problems regarding the selection of sites on the activation page.

First, if no changes were pending, the option "Select all checkboxes" in the "Changes" menu was still selectable.

Furthermore, the mentioned option was still "Select all checkboxes" instead of "Deselect all checkboxes" on page visit with pending changes (all affected sites are already selected on page visit).

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