Remove github fetch dependency on Grant page

[michaelwood]

It appears we fetch the 360Giving schema on loading the grant page in order to flatten the data into a table. This isn't particularly efficient and creates a dependency on being able to fetch that file from github. This could be fixed by either downloading a copy for the first load or not using the schema file.

HTTPError: 500 Server Error: Internal Server Error for url: https://raw.githubusercontent.com/ThreeSixtyGiving/standard/master/schema/360-giving-schema.json
(23 additional frame(s) were not displayed)
...
  File "grantnav/frontend/views.py", line 1262, in org
    return render(request, "org.html", context=context)
  File "grantnav/frontend/templatetags/frontend.py", line 76, in flatten
    return sorted([(key, value) for key, value in flatten_dict(d)
  File "grantnav/frontend/templatetags/frontend.py", line 76, in <listcomp>
    return sorted([(key, value) for key, value in flatten_dict(d)
  File "grantnav/frontend/templatetags/frontend.py", line 37, in flatten_dict
    schema = jsonref.load_uri(settings.GRANT_SCHEMA)