Re-style the "region placeholders" so they are not ugly.

Problem/Motivation

The yellow boxes signifying regions in "place blocks" were inherited from the demonstrate regions page and not specifically designed for their current purpose. They are also just plain ugly.

Proposed resolution

Since these areas denote in place editability they need to share styling and affordances with edit module and contextual links. The following image and attached video shows the updated styling integrated with edit toggle and off-canvas tray (not part of this issue).

Image links to attached video

Remaining tasks

1. Change/remove background color
2. Change button style to match contextual link
3. Add icon to button
4. Make hover, focus, active states match contextual links

User interface changes

The regions placeholders will have white backgrounds with some affordance that signifies "drop zone" or "area where things can be placed". The button style will change to match contextual links style with an icon.

API changes

None

Data model changes

None