Label list and label detail pages are still somewhat slow to load

[StephenChan]

Experiment more with pre-computing and/or caching here. We're seeing maybe 40 seconds for the label list if viewing for the first time in a day, and 10-15 seconds for Turf's label detail page, for example.

[StephenChan]

PR #403 computes the label popularities in an asynchronous task, and also reduces the label list's DB query count related to functional groups. When the changes are pushed to the production website, we should check whether the performance is good enough now.