Create more plausible category names in create_testdata

The create_testdata Django management command creates for us 70 different categories and 6 exams in each category.

This is great for local testing, but unfortunately they're named like "Category 1", "Category 2"... and "Exam 1 in Category 1", "Exam 2 in Category 1"..., etc. This isn't very helpful to actually simulate prod-like environments, especially around category and exam search.

Thus, this MR adds 70 deterministic plausible-looking category names (e.g. "Advanced Statistics", "Introduction to Parallel Datastructures", as well as exam names like "HS21"..."FS26".