Use the new django transaction API

fixes #1027
Merge pull request #1361 from ostcar/transaction

Used the new django transaction API.
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.
Refactoring.