New field grouping

The instance API has been updated to return all fields visible to the current user and an array of field sections defining the explicit grouping and ordering of fields in the mobile apps.

The fields the tree detail table view now use the two-line "subheading" style, so we have more room for detailed field labels.

I elimitated the no-longer-used fieldSections property on the Environment and replaced it an array of section titles parsed from the instance API response.