Show a spinner in lightbox when slow to load

[gnprice]

As @sstangl suggested in #2888, when the image in the lightbox takes a while to load,

It would be useful to:

• Display something more meaningful than a black screen -- possible a spinner, that a background download is in progress?

This'll be somewhat less common in the medium term as we roll out server-side thumbnailing, but still a fairly common case when people are on slow network connections.

This is a polish feature that would be nice to add.

[gnprice]

@borisyankov added, in a comment on #2888:

I was considering showing a low-rez thumbnail while the high-rez one loads, now that we support thumbnailing.

Both steps would be good.