Return stewardship w/ default mobile fields and search fields

Because this is adding a new type of field group that the mobile do not
have the ability to render yet, these changes are only available in v3 of
the API.

This is intended to be the final feature in v3, and all new breaking
changes should be part of v4.
I thought about defining them there, but it's tricky.  I'd prefer to not require every instance to have it's own defined set of `mobile_api_fields` until we have a UI for setting them, because in practice with the search fields it was horribly annoying needing to do a data migration every time we added a new field.

What I would prefer to do is to put the stewardship fields in `DEFAULT_MOBILE_API_FIELDS` and return to using `_make_config_property`, but the Stewardship fields are defined in the addons repo, so I can't (though they probably shouldn't be there).

One option might be to only add the collection UDF fields in if `mobile_api_fields` isn't defined, like this:
```python
if 'mobile_api_fields' in self.config:
    return self.config['mobile_api_fields']
...
# Add cUDFS to DEFAULT_MOBILE_API_FIELDS here
```