Refactor authz strategy
Embeds the authz in the same table
Introduces the Guarded interface
Requires db migration
Merge branch 'master' of https://github.com/slipstream/SlipStreamServer
Avoid NPE by validating entity.  Provide meaningful error messages to the user and log them as well.
Merge branch 'master' of https://github.com/slipstream/SlipStreamServer
Merge branch 'master' into issue_186