Refactor the app folder structure, to ignore the `/ext` folder

This changes the app folder structure to have everything related to the extension in the `/src` folder, and keep the `/ext` folder strictly for the compiled/built version.

This means we `gitignore` the `/ext` folder and not have to ever diff built/compiled files or edit anything outside the `/app` folder.

I've updated the Gruntfile to delete everything in the `/ext` folder on build, and copy the static assets from `/app` that were previously in `/ext` (`_locales`, `icons`, `pages` ..).