Clean up the circle/group/gallery sheets to look better on iPad

In #253 (closed) we found that the keyboard covers the user search interface on the group creation dialog.

So in this MR we (1) remove the ability to invite initial users and (2) clean up the remaining elements on the circle and group creation sheets, to look more like the photo gallery creation sheet. We also make the gallery creation sheet work better on iPad, by reducing the size of the avatar image.