Allow accepting/rejecting individual changes in a change request

[baszoetekouw]

A change request can contain multiple changes. Currently, the entire change need to be rejected or accepted as a whole.

Instead, allow an admin to select which of the changes should be accepted, and which should be rejected. Automatically inform the requester of which changes were rejected.

(continued from #440)

[baszoetekouw]

Complex too implement and doesn't occur very often. Possible alternative: split a change request in individual changes at the source. Could possibly be grouped.