Release version 2.4.0

Major features:
* Slides: A new content format without any answer options has been
  added.
* Interactive keynote: A new use case including the 'slides' format has
  been added. It replaces the 'All-inclusive' use case.

Minor features and changes:
* An overlay showing count of new student's questions and average
  feedback has been added to presentation mode.
* Student's questions and comments are now directly displayed instead of
  a list of subjects.
* JSON export and import now include session info and feature settings.

Bug fixes:
* Editor buttons now add new lines when necessary to produce correct
  Markdown.
* New lines in Markdown content are now displayed as is.
* All question formats apart from 'grid' are now correctly exported to
  CSV.

Changes for developers:
* Initial support for localization variations has been implemented.
This tag has no release notes.