Make shareabouts places editable in-browser

[goldpbear]

We'd like to give authenticated dataset owners/administrators the ability to edit and delete shareabouts places from within the browser interface.

Features

• Support basic editing: open a place detail view in editor mode, update title, body, and comments fields, and make a place invisible - #500
• Support editable attachments - mapseed/api#59
• Ingest existing landmarks data - mapseed/api#60
• Make lines, points, polygons, and geometry styles editable directly from the map - #410
• Support rich media features such as videos, html embeds, linkd, etc. - #407
• Support admin-only places - #498
• Support admin-ony form fields - #499
• Make it possible to add unused field types to a place model in editing mode - #497
• Editor "draft mode" - #501
• Support form-only fields - #523
• Support image uploads in the rich text editor - #525
• Preserve landmark url style - #549