Only work with open labs on /request page

This pr:

• only shows requests for currently opened labs.
• only counts open requests for these open labs (which was the main perf hit for @MrHug and @otto).
• shows request history for (closed) labs on the lab detail page for TA/managers/teachers, to preserve information accessibility.

@MrHug could you check if you are okay with this movement of data? We don't have a central filter place for it this way, we could re-structure this pr to make the requests page have an option to show all (taking the perf hit for those specific requests), but I'm not sure if it's needed