Do display "new point marker" for first marker in list (since polygon is closed)
Merge pull request #1283 from RickMohr/fix-django-tests

In tests fetching plot detail, pass `instance` argument to `make_request`
Distinguish between next/prev marker and next/prev point

* When getting next/prev marker, look in `_markers`
* When getting next/prev point, look in `latlngs()`