Form editor: fix map previews, and improve dragging

Summary:
- Fix map previews in form editor. Fixes T684
- Replace attributes with dummy el when dragging Fixes T842
- Show placeholders. Fixes T841

Test Plan:
Go to Manage Form -> edit a form
Drag some form fields around.
See placeholders
See just title in the element while draggable
See maps rendered correctly in previews

Reviewers: aMoniker, shadowhand

Reviewed By: aMoniker, shadowhand

Maniphest Tasks: T841, T842, T684

Differential Revision: https://phabricator.ushahidi.com/D398