Simplify form editor views

Summary:
- Move all the form manager stuff into a sub app
- Start moving logic to FormEditorController
- Move edit/save attribute handling into editorcontroller
- Drop availableAttributeList and just include it in FormEditor
- Rename FormAttributeList -> AttributeList

Test Plan: Go to http://ushv3.dev/settings/forms navigate around and check its all working

Reviewers: eyedol, shadowhand

Reviewed By: shadowhand

Subscribers: rjmackay

Maniphest Tasks: T681

Differential Revision: https://phabricator.ushahidi.com/D326