Write an iOS photo upload & share target client

[radkat]

This is moved from perkeep/perkeep#1741

[bradfitz]

Is this repo (named "photo-sync") going to be for both Android and iOS or do you want separate repos for the two clients?

[radkat]

I was thinking the same repo for both clients.

[radkat]

But now that we have perkeep/perkeep#1766, this becomes a plan B.