Importer performance

[richardebeling]

Best reviewed by commit. Fixes #1840.

Openpyxl no longer spends half a minute on determining the worksheet dimensions.
We prevent some N+1 query behavior, making the importer and the staff semester page (which the importer redirects to) a bit faster.

On my machine, import times for a 6000 row sheet go from 32-40s to 3-6s.

[richardebeling]

moving the staff semester page N+1 query fix to #1842

[niklasmohrin]

haven't measured or thought about the prefetches yet (more prefetching = more performance, right?); I generally trust you with this, but if you want I can also take a closer look