Enable issues "pagination" dropdown. Fixes #372.

Enable the "issues per page" dropdown. Three choices, 25, 50 (default), 100 (max allowed by GitHub).

And add some functional tests that makes sure clicking opens it, clicking an option updates the label, and selecting a choice re-renders the view with the new amount.

r? @karlcow for the python bits
r? @magsout or @tagawa for JS (if either have time, I know it's a lot).

Thanks!
Will probably tackle this when enabling the sort dropdown #371.