fixes to support latest rjr/json gems

Mostly to use new safe json parsing mechanism
fix to manufactured::create_entity: delete location if created but entity could not be added to registry
swap reordering of system / target location retrieval in manufactured::move
update / remove some comments
add User#remove_role and Role#remove_privilege methods
flush out users::remove_role and users::remove_privilege rjr methods
resolve fixme to remove privs upon entity deletion when moving to a remove server
update some comments
flush out a few specs

Small code changes for testing purposes
Add a resource preload mechanism to the frontend ui

Simply specify resource to preload when initializing the
ui and they will start to be retrieved immediately so that
when it's time to display entities on canvas their graphic
representations are good to go
flush out a few more tests