Remove navbar cache

[richardebeling]

Removes the navbar cache.

Should thus fix #1645. Increases page load times by ~20ms. I tried

1. just fetching semesters from the DB once instead of twice with different filters (still in this PR)
2. prefetching the groups of request.user to allow the privilege-checks to happen without database hits.

Both didn't really change much about the performance. At 40-60ms to render a static view, the effects of removing singular db hits is really hard to qualify. I left the semester logic in, as it is contained, and reverted the user profile groups stuff, as prefetching is a bit tricky to do there: We only want it for request.user and not for other users, and ideally, we'd want it directly during initial creation of the object (and not later annotated by a middleware).

[niklasmohrin]

Looks good to me