Add URL tests

We want to test that every URL succeeds (200) or fails with bad data (404).
We now have such URL tests for our "root" URLs (defined in opentreemap/urls.py) as well as our "app" URLs defined in:
    ecobenefits/urls.py
    geocode/urls.py
    treemap/urls.py

I did not add tests for URLs defined in:
    api/urls.py
    registration_backend/urls.py

Notes:
* I removed a few existing URL tests from other test files and added simplified versions to treemap/tests/urls.py
* I removed the assertTemporaryRedirect() helper and used django assertRedirects() instead
* treemap.views._plot_hash() now raises a 404 if instance lacks plot
* treemap.views.boundary_to_geojson() now takes an instance argument and raises a 404 if instance lacks boundary