Implement basic frontend

The frontend shall be structured like this:

• A display groups view as single entry point
  • List of display groups
  • Possibility to add and edit display groups
• A detail view for one display group
  • List of displays
  • Possibility to add and delete displays
  • List of content slides
  • Possibility to add, edit and delete content slides
  • Possibility to re-arrange content slides
  • Layout: Probably two cards next to each other in the first row (Info about display group left, list of displays right), and list of content slides below that