Added Django's application configuration.

Used customized AppConfig classes in openslides.core, openslides.account,
openslides.projector, openslides.agenda, openslides.motion, openslides.assignment,
openslides.users, openslides.mediafile, openslides.config.

Refactored loading of signals, template signals and slides.
Merge pull request #1369 from normanjaeckel/AppLoadingSystem

Added Django's application configuration.