[Tags] Improve usability

[x] Fix class statistics links, tag is included in the url but it has no effect
[x] Change tag filter in class statistics so that it's possible to include and exclude tags
[x] Find out whether associated start year has any effect on anything or not
[x] In the class statistics view, only display students' tags of the current study programme (a student might have tags for other programmes as well)

the exclusion here:

UniversityOfHelsinkiCS / oodikone