Filter for study docs
Implement new, more useful filters. Use the same as in vvz.ethz.ch. Meaning for MAVT Bsc: Basisprüfung Weitere Fächer des Basisjahrs Block 1 Block 2 Block 3 Wahlfächer Fokus Fächer (with sub-cats) Analogous for ITET
Since some Lectures are in several categories, make a "tag" system, where a lecture can be tagged with all categories it is in. This way one can just go to his category and always find his lecture.