Refactor selection of boards

[PhMemmel]

Currently jumping from one board to another view can be not easy to understand.

I suggest to use a dropdown containing all available boards, sorted like for example:

• Public board
• Group boards
  • Group A
  • Group B
  • Group C
• Personal Boards
  • Student X
  • Student Y
  • Student Z
• Template Boards
  • Template for public board
  • Template for group boards
  • Template for personal boards

Also the templating could just be moved to a "template editor" which will be easier to understand as workflow for instructors. However, I suggest to not remove the "save as template" possibility, there just needs to be a selector what template to overwrite.

When this ticket is being worked on, the "dirty hack" from #20 should be removed again.