Enable the creation of new custom forms, refs T440

Summary:
- add custom form modal for edit/create
- update api to ensure every form has one group
- sync form field edits with preview

Test Plan:
- enable `form_wizard` feature
- go to custom form screen, add a new form
- edit the form after it is created
- add a field, save the field, see it change

Reviewers: eyedol, rjmackay

Subscribers: bodacea, srutto

Maniphest Tasks: T440

Differential Revision: https://phabricator.ushahidi.com/D305