Long photomap captions need scroll support

[mvl22]

Photomap items with longer captions, like

/photomap/8492/

need to be scrollable (or expandable in some way).

[mvl22]

c8a7a82 adds overflow: auto, but this does not fix the problem when using a touch screen.

[mvl22]

Problem reduced by maximising caption space in 5610ea5, converting layout to flex.

[mvl22]

Scrolling (with mousewheel) fixed in 19cd895.

[mvl22]

The caption still doesn't seem to scroll on iOS or Android. No obvious reason why.

Scroll wheel on desktop works as normal.