Allow framing of the embed and fire activity pixel periodically.

Since the websocketpocalypse, you no longer need to refresh to get new updates. This means the static pixel is no longer sufficient to keep the activity tracker up to date.  The client will now periodically fetch pixels to keep the activity count accurate.

A couple of tiny other fixes include allowing framing for the embed (oops) and adding a comment about the worker joining thingy.

:eyeglasses: @chromakode