Reading a blueprint wasn't checking to see if it had been deleted so it
was returning the most recent commit before it had been deleted. This
allowed things like starting a compose with a blueprint that technically
doesn't exist.
One exception to this is the /changes/ route, it must be available so
that you can use the commit hash to undo a delete.
This also adds tests for the various operations.
(cherry picked from commit d32f477e0b)
Resolves: rhbz#1683441
b399076cb0