Detect and report out-of-sync migrations via the API

When the Razor code and database migrations are out of sync, significant
problems can occur.  This adds code to detect when this situation has shown
up, report it, and refuse service to all callers.

For the client this will result in an error message to the user, including
instructions on how to fix the problem.

For MK images, etc, it will result in an error that they will observe and
retry in the near future; this is the best we can do, since we can't be sure
that the changes have not altered the behaviour they should undertake.

https://tickets.puppetlabs.com/browse/RAZOR-158
Signed-off-by: Daniel Pittman <daniel@rimspace.net>