[PR] Improve column editor experience in the Builder template

- [x] All TinyMCE editor instances load with the same set of options.
- [x] Do not restrict columns to 50% width, every TinyMCE editor fills 100%.
- [x] Add column numbers to aid with sorting once in an expanded view
- [x] Add a toggle to each column to hide/show content while working on large areas
- [x] Consider preserving state of the column when saving a page so that toggled areas stay toggled.
- [x] Look for ways to consolidate the admin template for columns
- [x] Finally add a `thirds.png` to avoid endless 404s.