Release version 2.6.0

Improvements:
* Guest authentication is now retained when logging in and will be
  restored after logging out.
* Questions and slides are now automatically released for answers/
  comments after creation.
* Forms for question creation/editing no longer show settings irrelevant
  for the question type.
* The solution can now be shown for free text questions.
* The statistics button now appears immediately after answering free
  text questions.

Changes:
* Switched from serif font to sans-serif font.

Bug fixes:
* The correct answer is no longer revealed by order for yes/no questions
  imported from CSV.
* Pressing <ENTER> in the question's submit field when editing no longer
  reloads the web app.

Removed features:
* Import and export for arsnova.cards and arsnova.click formats has been
  removed.

Known issues:
* CSV import/export does not work for flashcards.
This tag has no release notes.