fix: post request works in /calculate and Grades was exchanged by ArrayList(Grading), whoop uhm.. whoop!

refactoring: simplified package structure (no servlet/tag folders needed)
documentation: added some comments to the service classes

refactor: used jersey seeOther method for redirect instead

feat: implemented Servlet Filter (for all pages in /pages/ user is redirected to login if he is not authorized)
feat: implemented TagLib for hidden fields (showing the user name based on token in url)
feat: copied styles from groupfinding project

This class represents an annotation object

fix: fix variables in Feedback Interface

TODO: bump java version to 1.8 for Instant to work