Merge pull request #80 from VictorLowther/add-referential-integrity

+2 Add real referential integrity to our database.
Make database seed loading really fully transactional.
Simplify enforce_acyclic when creating role_requires.

Instead of manually iterating over all the role_requires to find
conflicting ones, fix up the all_role_requires view to have the
functionality we need to do it in one shot.

If we find a cyclic role dependency, we will spit out all paths through
the role dependency graph that would be made cyclic.