[WIP] Add shim for codemirror on touchscreens

**Note** I think the code is ready, but this needs some CSS moved from editor.hbs into ghost-ui, and cleaning up & also needs a review of the 'coming soon' message cc @JohnONolan 

fixes #2385

- stolen the CM shim from js-bin
- if we're on a touchscreen device, don't use CM
- if we're on a touchscreen device, show a coming soon message for uploads