feat: add update for user database entry

adding .idea folder to .gitignore for each subproject, because it's only folder for IDE configuration

It only contains IDE configuration which shouldn't be in a repository

test: adding unit tests for endpoints

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