feat: implemented role for user (isStudent true/false)

...in/java/unipotsdam/gf/core/management/ManagementImpl.java

...rc/main/java/unipotsdam/gf/core/management/user/User.java

...ipotsdam/gf/core/management/user/servlets/CreateUser.java

...ipotsdam/gf/core/management/user/servlets/UserExists.java

...dam/gf/core/management/user/servlets/UserHttpServlet.java

...chen/src/main/java/unipotsdam/gf/interfaces/Feedback.java

...tsdam/gf/modules/peer2peerfeedback/Peer2PeerFeedback.java

...amforschen/src/main/webapp/core/assets/js/showProjects.js

gemeinsamforschen/src/main/webapp/register.jsp

gemeinsamforschen/src/scripts/dbschema/fltrail.sql

gemeinsamforschen/src/scripts/dbschema/fltrail_users.sql

...rc/test/java/unipotsdam/gf/interfaces/ManagementTest.java