Changes a photo's details. Send any of title, description, photo_type and tag_list inside a
photo object; fields you leave out are unchanged.

• tag_list replaces the photo's free-form tags; the photo-type tag is kept.
• photo_type changes the type, and the tags change with it.
• Once set, taken_at, latitude and longitude cannot be changed, because they record where and
  when the photo was taken.

The token's role must allow access to all records, or the user must be the one who uploaded the
photo.