Additionally, remove unused loggers.

* Remove unused code
* Optimize imports (group 10+ and static imports)
* Remove redundant variable assignments
* Replace explicit type parameters with <>
* Remove redundant catch blocks
* Use varargs instead of explicit array instantiation
* Weaken visibility of methods and fields
* Simplify conditions
* Do not use StringBuilder for simple concatenations
* Fix code style
* Remove invalid Javadoc

Configuration for Socket.IO has been refactored to support proxies and
replace ambiguous property names:
* Added API property `socketioPath` to `/configuration/`
* Added property `socketio.proxy-path`
* Removed property `security.ssl`
* Renamed properties:
  * `socketio.ip` -> `socketio.bind-address`
  * `security.keystore` -> `socketio.ssl.jks-file`
  * `security.storepass` -> `socketio.ssl.jks-password`

The default port to bind to has been changed from 10443 to 8090 since
SSL is not enabled by default. There is enough room (8081-8089) for
additional severs (e.g. `grunt run`) in development environments.

The security.*.enabled properties are now loaded as booleans.

See also:
003b064b

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.

remove spingfox for ApiIgnore direction
use swagger-maven-plugin to generate swagger.json before compiling rather than
 dynamically generate it on request

Renamed optional.arsnova to ui.arsnova-slogan and made comments start
with an uppercase letter.

Updated copyright year and adjusted header format to be consistent over
all files and with other ARSnova projects.