Ensure we have decent mobile styles

[lmorchard]

From acceptance criteria:

• It should have a minimal, pleasant mobile experience that lets users know it's a desktop experiment.

[youwenliang]

Here's an idea for the mobile page: (Need to think about the content)
https://mozilla.github.io/testpilot-assets/ThemesRFun/01_Measurements/Mobile_page/
Icon: https://github.com/mozilla/testpilot-assets/tree/master/ThemesRFun/01_Measurements/Mobile_page/assets

We could also add a call to action button that links to testpilot's experiment detail page.

[johngruen]

This looks good to me, i can pick this up.

[pdehaan]

@johngruen Is this still legit, or did you fix it all w/ PR #188?

[johngruen]

No we should still do something for mobile browsers