Removes border radius from avatar to match mockups
Adds off-white as a new color variable
Removes app.css as a layout-specific stylesheet

- moves most classes to column.css.
- Updates conversations to take advantage of this

We might want to make this even more modular later.
Alters heading styles based on account settings design
Updates markup for account settings design
Removes app.css
Removes the docs layout specific CSS in favor of column.scss

Uses CSS and classes from columns across the style guide and app for
consistency. Removes docs.scss.