Fix diagram creation

MSP-9816

Diagram creation did not work because create is implemented in RailsERD
by using class-level callbacks and those callbacks are not inheritted,
so I copied RailsERD::Diagram::Graphviz's callbacks into
Metasploit::ERD::Diagram; added specs for callbacks and create; and
ensured that the save callback creates the parent directory so the
default save from RailsERD::Diagram::Graphviz won't raise an exception.