Change instance fields schema in API

This changes accomplishes two goals

- Supports explicitly defining the order in which fields appear
in mobile apps

- Makes the instance API more useful beyond our mobile needs.
All readable fields are now returned with the instance, not
just the ones used by the mobile application.

This is the new, expected schema of the the mobile_api_fields
value in the instance config

[
  {
    'header': 'Tree Information',
    'field_keys': ['tree.species', 'tree.diameter', 'tree.height']
  },
  {
    'header': 'Plot Information',
    'field_keys': ['plot.width', 'plot.length']
  }
]
Update form tag to support MultiPolygon fields

There are now green infrastructure fields that have MultiPolygons as
their type.
Merge pull request #1253 from jwalgran/topic/change-instance-info-api-schema

Change instance info API schema