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

* 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

Added support for an activation webpage and renamed dialog config param
for consistency.

The API returned "ldap" instead of "guest" for guest authentication
before.

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.

This should perform much better.

Socket.IO about changed active user count.

prevent duplicates if the client uses Socket.IO and still sends the
**/online ping.

server.