[JENKINS-21970] enable checkbox on rebuild

Fix for [JENKINS-21970] (https://issues.jenkins-ci.org/browse/JENKINS-21970).
When displaying a rebuild page with a matrix-combinations parameter, I cannot toggle combination checkboxes as they are in anchor tags and the browser opens a new page.
This pull request moves checkboxes out of anchor tags.

If this is an intended design to have users not to change combinations, please decline this request.