Merge pull request #890 from emanuelschuetze/translation-pt

Added Portuguese translation
Prepare OpenSlides master for the next major release
Removed utils from INSTALLED_APPS.

Moved utils.management and utils.templatetags to core
Added template filter 'absolute_url'
Fixed a bug when setting a config var before getting one
Deletes config cache after each test if it exists.
Use GenericForeignKey for agenda related items, fix #865
Add default sorting of widgets. Fix #493
Fixed repr in widget
Add prototype of full text search integration in openslides
Allow for empty title and text body, fix pep8 errors
Full text search with haystack and whoosh.