gnome-photos: String freeze break request (GNOME/gnome-photos!28)

Hey,

I'd like to add three new user-visible strings to Photos to make the
UI for importing content work well with different kinds of devices.
Testing with SD cards has revealed that some of them may not present a
decent user-visible string. Therefore, it's necessary to find an
alternative title for the header bar.

The strings and accompanying screenshots can be found in this merge
request:
  https://gitlab.gnome.org/GNOME/gnome-photos/merge_requests/28
 
I was hoping to send this request a few days earlier, but I ran into
an unexpected snafu with internationalizing strings with plural forms
across different contexts. Therefore, it took a little more effort on
my part than is usually required for such changes. Sorry about that.

Cheers,
Rishi