Tablet layout visual refinements

[iamthomasbishop]

This issue covers various refinements related to layout/alignment.

Visualization for reference:

Punchlist

• User should be able to scroll by dragging anywhere on canvas
• Center column container should have max-width of 580, which provides a nice readable balance/line-length
• Block boundaries should extend beyond center column by 16 on each side (inner block 580 to edge of container)
• Quick Toolbar should extend to edges of canvas

Note: Upon further inspection, the container-width might be dependent on readable-margins, so this is probably fine as-is. At least on iOS, when I change the OS-level font-size the container width adjusts (I didn't realize I had my OS-level setting to lower than default). However, the font-size doesn't actually adjust in the editor – I will create another separate issue for this.

[etoledom]

Related: #473