[REQUEST] Design mobile behaviors

## Suggestion/Concern

This issue is looking to define what behaviors should be in the mobile space as it relates to Theme Builder. It would be great if designers could consider how Theme Builder's user experience may be different on a tablet. For example, what should be done with the upper navigation? This issue could also be used to design mobile versions of our templates.

## Proposed Solution

No files, tests, or entry points are named. Start by reviewing the Theme Builder experience on tablet and mobile, especially the upper navigation and mobile versions of templates; done means the mobile behaviors and template designs are defined for the relevant screen sizes.

Written by the indexing model from the issue text.