Create A/B test for auto-opening share panel

[ianb]

Note: requires #2077

In this experiment we will either not open the sharing panel on shot creation (the current behavior), or open the share panel (older behavior we can restore).

We want to track if shots with the share panel auto-opened are shared more than other shots. To do this we may have to track the variable in the shot itself. Perhaps this means that we should encode the author's A/B variables in the JSON body generally. Then when someone views the shot, we change the event somehow based on the author (at least in this case).

[ianb]

Note the commit that turned this off is d4964a7