Think about how improve AppHandlerRegistry and document it

[AMontagu]

AppHandlerRegistry interface may be improved in term of usage.

But all this magic behavior should also be explained/documented so people that know how grpc python work understand the way DSG is making the connection.

• Delete old usage code

[AMontagu]

Idea:

• Use the dajngo settings that register app to introspect the ones compatible with DSG (should be withtjout specific add)
• Settings for the ROOT_HANDLER may then be duplicate if we can automate it by the django settings
• Use the handler.py to declare Service in the django admin way and not calling a method.