Provide an interface for creating instances in ui tests

The regular site can simply call 'make_instance', however, if addons are
installed the restrict or change how instance creation is done it breaks
the workflow.

The solution used here is to override the instance creation in settings.py.
Add in basic map tests for adding trees
Merge pull request #785 from ahinz/topic/add-on-map-page-selenium-tests

Topic/add on map page selenium tests