Release version 2.6.0

Features:
* Experimental support for CouchDB 2 has been added. Note: The data
  migration script is not compatible with CouchDB 2 and has to be run
  before an upgrade.

Improvements:
* Error handling and logging has been improved. It should now be easier
  to find the cause of problems. API error responses now contain the
  name of the `Exception` which caused the error. Further details for
  debugging purposes can be enabled with the new
  `api.expose-exception-messages` setting (Do NOT enable in production
  environments!).
* Updated OAuth handling to restore compatibility with 3rd-party login
  services.

Bug fixes:
* Fixed multiple bugs caused by incorrect type handling in the database
  layer.
* Fixed XFO header check behind reverse proxy (used by clients when
  embedding external websites).
* Fixed rounding error in learning progress calculation.
* Fixed `security.cors.origins` setting.
* Fixed import of data from older versions.

Security:
* Fixed DoS vulnerability in authentication handling behind reverse
  proxy.

Configuration changes:
Minor changes to the web server and Tomcat proxy configuration are
required (see [installation guide](src/site/markdown/installation.md)).
This tag has no release notes.