Events for all learning progress related actions
are generated, like answering a new question.
The event classes are used as a tag to evict
the cache based on the session.

A NotFoundException was mistakenly thrown when the Session was set to
null via Socket.IO. Additionally, the user count was sent before it
had been updated.

count to NewAnswerEvent websocket broadcast. Task #14471

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

#14099

* Organize imports
* Add missing Override annotations
* Remove trailing whitespace

SessionController: Append integer to a string will result in a string
ApplicationPermissionEvaluator: Code formatting
UserService: Create new user object instead of assining parameter reference
ARSnovaSocketIOServer: No exceptions are thrown

The removed implementation used events wich were never published in any
part of the application.

This patch intends to close all websocket connections on bean
destruction of ARSnovaSocketIOServer.

This reverts commit c22330fd.

This enables access to all UserSessionService objects from within the
application. This could be used to send events to all session users.

This patch uses an in memory storage of logged in users. Every time
users send a (online) heartbeat the last known active timestamp of all
users will be checked and inactive users will be removed from active
user list.