Release version 2.3.0

Major features:
* Improved LDAP authentication support: Additional settings for LDAP
  search and a privileged LDAP user have been added.
* Usernames for admin accounts can now be set up in the configuration
  file. These accounts are privileged to create global "Messages of the
  Day". Additional privileges might be added for them in future
  releases.
* Splash screen settings have been added to override the frontend
  theme's defaults.
* The API has been extended to support features introduced with ARSnova
  Mobile 2.3.

Minor features and changes:
* Markdown formatting, learning progress, student's own questions and
  the question format flashcard are now active by default and can no
  longer be disabled for the whole ARSnova installation.

Bugfixes:
* The `security.authentication.login-try-limit` setting now works as
  intended.

Changes for developers:
* API documentation is now exposed in Swagger format.
* Startup time of Jetty has been significantly reduced.
* Version information is now saved with builds and exposed by the API.

**This version is brought to you by:**
Project management: Klaus Quibeldey-Cirkel
Lead programming: Andreas Gärtner, Daniel Gerhardt, Tom "tekay"
Käsler, Christoph Thelen
Contributions: Eduard Ellert, Tjark Wilhelm Hoeck, Mohamed Sami Jarmoud,
Stefan Schmeißer, Paul-Christian Volkmer
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