Allow breakpoints in canvas to be customizable

Overview

Currently the breakpoints in the preview are set values (see attached image). The preview window also does not allow for resizing below 1024px.

Proposed resolution

User interface changes

I'd like the breakpoints in the page builder/preview to be customizable, so that we can set them to match our current design system as needed. I'd also like to be able to resize my window as needed in the preview.