Guides - Getting Started 3: add alert to code snippet handling 'invite' event

This makes it more consistent with the JSFiddle and has better UI, IMO
Guides - Sample Applications - Telepresence: fix caller filtering snippet

It now actually works and is more consistent with the JSFiddle.
Guides - Sample Applications - Telepresence: fix snippets to match jsfiddle

they now actually work when pasted together