Clarified behavior in tests

- What happens when you are on page 10 and the superset changes to only
  have 5 pages? Or there was only one model on the page and one was
  removed?

  I wrote new tests for this case. The first idea was having the
  pagination reset to page 0 on reset, but this is a surprising and
  unintuitive reaction (imagine changing sorting and the page changes).
  So now we try to keep it on the same page or the last page. If you
  were on page 7 and now there are suddenly only 5 pages, you're now on
  page 4 (the last page since we count from 0);

- Also decided that (for now at least) we'll just always reset the
  pagination on model add / etc instead of adding / removing. In the
  future I'll explore using .set instead of .reset to get the simpler
  result.