Play / Edit toggle button in Editor

Added play / edit toggle button to the editor.
Fixes #310 

STT:
* Open an app in the editor
* Play / Edit toggle appears on bottom of app as pictured here https://redpen.io/yq6334167211aa1739
* Tapping on the non-selected option switches the editor and the toggle button to that mode

I didn't do any any animations on this toggle since we're reloading the page anyway - will do for places where we're on one view.
Can you change this to `Preview` (and edit the locale json file)