Merge branch 'motd' into 'master'

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