Add a controller `OfflineMainCtrl` to enable interactions in offline mode
Create a constant for pages that require connectivity and apply that in app.js
 - Create the constant requireOnline.js
 - app.js adds a check to determine if site is online or not
 - each page that requires connectivity now checks for it (same as enforceAuth)
 - attempting to access page that requires connectivity while offline redirects to /offline
add cache manifest and reference it in index.html