Handbook update for weighting & Periscope - Source Code

Weighting with our current process during 12.5 went very quickly, because @nick.thomas and @reprazent hardly disagreed. It did not seem like it was a 2 person job. This MR changes the weighting process to hold 1 person directly responsible for weighting during a release, and clarifies that the assignee is ultimately the final step in an issues weight.

Thoughts? @nick.thomas @reprazent @oswaldo @patrickbajao @igor.drozdov @kerrizor

PS - This also embeds our Periscope chart into the handbook.