- add flatpages app to gnuviechadmin.settings.base.DJANGO_APPS
- add imprint handling to gnuviechadmin.context_processors.navigation, remove
unused about page handling
- add URL 'imprint' to gnuviechadmin.urls
- replace link 'about' in template base.html with 'imprint'
- add templates for flatpages
- add german translation for imprint and contact navigation links
