Rework approval queue

[douglm]

The approval queue currently shows the events in much the same way as the manage events display - i.e. a list of instances or single events.
I think it should show only single events and the master. Clicking on that will allow update of the master or single event and approval.
There should also be an option in the initial display to show any overrides. This allows the approver to check there are no unexpected changes to appear in the future.
We could also provide a way to select a particular instance and update it to create an override - though that could be handled after approving.
Comments? Also this is a lot of work so not sure when...