fix bug where abstention questions are not exported

fix bug where not all attributes from grid questions are exported

implement: via two booleans you can now decide whether AnswerStatistics and FeedbackQuestions shall be exported, too

add empty test methods

add constructors to transport classes to parse "main"class

This fixes a potential NPE and should slightly improve performance.

See also:
003b064b

Also added documentation for the property.

 - add controller/service/entity/dao-functions
 - add motdlist so that users can tell the backend, which motds they have read
 - add admin account (in arsnova.properties: security.adminaccount).
 - add permission function for admin

Version information is now compiled as part of a build and written to
version.properties. It is exposed by the API.

Markdown and learning progress features are no longer considered
experimental. Leaning progress can no longer be globally disabled.

Added 'ui' prefix for splash screen settings.

Feedback reset will be disabled for liveClicker-sessions.

An LDAP search pattern can now be used instead of binding with a DN
pattern.

This reverts commit fc78eb18.

SASS variables only exist at compile time and are not available for
arsnova.properties.