Add MultiPolygon Support to Submission Shapes

[rajadain]

Overview

Currently, a submission is only allowed to have simple polygon shapes:

iow-boundary-tool/src/django/api/models/submission.py

Line 17 in 100e8d0

shape = gis_models.PolygonField(geography=True)

However, a number of real world utility boundaries are complex multipolygons. While we may not support drawing complex multipolygons at this stage, we should still allow users to upload shapefiles or geojsons created elsewhere. To store this, we'll need to change this field to be a MultiPolygon.

Also update any test data and tests to match this.