Custom form wizard, fixes T136

Summary:
- adds a `custom_forms` feature
- "Manage Forms" added to Workspace menu, feature protected
- edit existing forms
- change name (label) of form fields
- add form fields via drag and drop

Not done:

- needs better styling
- edit field descriptions
- save form field order
- anything to do with validation

Test Plan:
- enable the `custom_forms` feature
- click the Manage Forms link in the menu
- click a form to edit

Reviewers: dkobia, eyedol, shadowhand

Subscribers: dkobia

Maniphest Tasks: T136

Differential Revision: https://phabricator.ushahidi.com/D119