Upgrading Netty-socketio makes ARSnova compatible to newer versions of
the official socket.io client.

This fixes a potential NPE and should slightly improve performance.

See also:
003b064b

Also added documentation for the property.

MotD Feature:

There are two types of MotD: 
AdminMessages
SessionMessages

Every Message has a title, text, start- and enddate.

You can declare an admin account in the arsnova.properties (security.adminaccount = <adminusername>) and with this account you can create Messages for different audiences (different types are: all (shown on startpage), loggedIn, students, tutors) which will be shown in a messageBox.

SessionMessages can be created for every session by the sessioncreator and is displayed, when you enter the session.

When a user that is not logged in via Guest, he pushes a list to the backend with all the messagekeys he read and acknowledged, so that he isn't shown the same messages over and over again. In addition to that, the keys are also stored in the localStorage (e.g. for guest accounts).

See merge request !15

Diabled Checkstyle's Checker.TreeWalker.RedundantThrows module which no
longer works with plugin version >=2.16.

 - add controller/service/entity/dao-functions
 - add motdlist so that users can tell the backend, which motds they have read
 - add admin account (in arsnova.properties: security.adminaccount).
 - add permission function for admin

Version information is now compiled as part of a build and written to
version.properties. It is exposed by the API.

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.

Upgrade to Spring IO 2.0.1.RELEASE

Add "liveClicker" use-case and possibility to lock live feedback

### Live Clicker
An additional use-case called "liveClicker" has been added (https://scm.thm.de/redmine/issues/17162). It uses the live feedback implementation in order to make the voting in the style of hardware clicker  systems possible. The timed deletion of feedback votes has been disabled for sessions with "liveClicker" configuration.

### Feedback lock
The feedback-lock feature allows the speaker to lock the live feedback/clicker. While the lock is active, no further votes are possible. On lock removal all votes from the session will be reseted.



See merge request !13

Feedback reset will be disabled for liveClicker-sessions.

Add support for LDAP manager and search

New configuration settings have been implemented to allow additional
LDAP environments:
* `security.ldap.user-search-filter`
* `security.ldap.user-search-base`
* `security.ldap.manager-user-dn`
* `security.ldap.manager-password`

See merge request !12

An LDAP search pattern can now be used instead of binding with a DN
pattern.

This reverts commit fc78eb18.

SASS variables only exist at compile time and are not available for
arsnova.properties.