Add ability to set mobile as default viewport

[marcusforsberg]

Is your feature request related to a problem? Please describe.
With the majority of web traffic occuring on mobile, we always encourage our clients to check the "mobile" viewport in the Presentation tool. With certain sites receiving around 85-90% mobile traffic, we'd like to have the Presentation tool always default to the "Mobile" viewport instead of the "Desktop".

This would save editors the extra click and also help remind them to actually check the mobile viewport, where currently some of our editors are stuck in the desktop-first or desktop-only mindset.

Describe the solution you'd like
I think an option on the Presentation tool would do the trick, say defaultViewport.
If unset it would default to desktop being the default as it is today. This gives the option to customize this on a per-project basis:

 presentationTool({
    // ...,
   defaultViewport: 'mobile'
 },

Ideally, it would be great if the user's preferred viewport option was also saved in local storage, as is the case with the pane sizing today.

Describe alternatives you've considered
We're currently doing some hacky router state sniffing to try and make this work, but it's not ideal and doesn't cover all scenarios.