Responsive grid-based page editor

[jbroadway]

See branch:

https://github.com/jbroadway/elefant/tree/grid_based_page_editing

Remaining to-dos:

• Drag to reorder rows
• Duplicate row
• Collapse columns
• Versions of grid (comparison)
• Remove page body from add/edit forms if enabled
• Use Redactor external toolbar to not obstruct buttons on narrow cols
• Make grid-based pages cacheable
• Make a grid-based theme with basic row styles (inset, light, dark, etc.)
• Theme should list valid column units (in elefant.json file?) to control consistency of widths within site

I believe that's everything that's left.

[jbroadway]

The new blocks/group helper solves most of this now, so I'm closing this and retiring the associated branch's approach in favour of making improvements to blocks/group instead.