Adds model name to AdminModel2Mixin template context.

All views subclassing AdminModel2Mixin will need the model
name in their template context, so moving it to the base
class so child views don't all have to override
get_context_data.
Merge pull request #115 from chrislawlor/feature/template-context-refactor

Adds model name to AdminModel2Mixin template context.