feature: DjangoLDPAdmin and DjangoLDPUserAdmin

• Moved automated admin registration to app configuration (made default configuration)
• Removed depreciated create_local_source
• Added DjangoLDPAdmin. This acts as the source of truth for Model admin class dependencies (for now inheriting GuardedModelAdmin dependency and will inherit future i18n dependency)
• Added DjangoLDPUserAdmin. This overrides Django's UserAdmin and should be used by clients in its place. Django's UserAdmin overrides get_fieldsets to support add_fieldsets, and so DjangoLDP ensures that federation fields are included in these forms unless explicitly excluded
• Updated README, but I'll also update the docs in the Wiki when this is merged

mentioned in merge request djangoldp-account!62 (merged)

mentioned in issue applications/hubl#272 (moved)

merged

mentioned in commit 7673a59b