Limit photo selection in gallery screen in Default mode

[albullington]

If a user is in Default mode, they should only be able to select a single photo from the gallery screen, via the AICamera. From there, they'll proceed to the Match screen. The goal of this change in flow is to limit the ability for a brand new user to save/upload an observation that has photos of many different organisms, and to teach them through doing that an observation should contain a single organism.