Issue #188: Admin: highlight fields with errors

Not looking for this pull request to be accepted since my tests are not passing, but the styling works whenever Rails creates a *field_with_errors* DIV.

I noticed tests associated with blank forms for [creating organizations](https://github.com/codeforamerica/ohana-api/blob/master/spec/features/admin/organizations/create_organization_spec.rb) (Rb 25) and [creating location](https://github.com/codeforamerica/ohana-api/blob/master/spec/features/admin/locations/create_location_spec.rb) (Rb 18) may be passing in error. The corresponding models may need additional validation, but I'm curious as to why the tests are passing.

I'll submit another GitHub issue separately.