Enable setting default power level when creating gallery (and circle?)

We got a request from a user who wants to share their photos, but they don't want their friends to post in the shared photo gallery. They could use #282 (closed) to change the default permissions after they create the gallery, but they argue (correctly I think) that most people won't ever bother to do that until it becomes a problem.

The user wants all galleries to set the default power level to "Can View", and they make a reasonable case for this. Some user could be a jerk and put a bunch of junk in your gallery. OTOH I think Apple iCloud Photos shared albums allow everyone to post.

So it seems maybe the solution is to punt this question to the user. When they create a gallery (and also a circle???), let them choose what the default power level should be.