buildrequest collapse

 - TODO:
   - update master/docs/manual/customization.rst

First iteration of the project.
I plan to update customization.rst.
Agreed regarding `completeBuildRequests`.  And let's get collapsing working without concern for buildsets to start with, but I think that in the final analysis if buildrequest a, in buildset A, is collapsed into buildrequest b in buildset B, then buildset A shouldn't be considered complete until buildrequest b is.  That will require some modifications to a the database schema.  But can happen afterward.