Twenty Thirteen: Captions cover image links in galleries with many columns

[kwight]

In single views, the fixed height of gallery captions makes image links inaccessible in galleries with seven columns or more. The problem also happens with lower numbers of columns as screen width narrows.

[kwight]

Patch 23769:

• hides captions for galleries with 7 columns or more
• hides captions for galleries with 4, 5, or 6 columns at the 643px breakpoint
• ensures captions are never more than 50% of their image

The 50% max-height makes sure that there is always clickable space for the image link, which becomes more important on touch devices with smaller physical screens.

[lancewillett]

Discussed in office hours:

• Yes, hide for 7 columns or more
• Hide for all columns in smaller screens (any touch device, basically, phone or table won't be able to see the captions anyway)
• Try to style the captions so that one-word, one-sentence captions don't need to take up as much height

[kwight]

23769.2.diff:

• remove captions on all 7, 8, and 9-column galleries
• remove captions for all galleries at 767px and below

[lancewillett]

In 23700:

Twenty Thirteen: hide gallery captions from small screens and when a gallery has 7 columns or more. Props kwight, fixes #23769.