Twenty Fifteen: Move image caption right after the attachment on image attachment page

[iamtakashi]

Current:

Like other default themes, the image caption should come first and followed by the description.

[iamtakashi]

Swap the order of caption and description on image attachment page.

[iamtakashi]

After:

[iandstewart]

In 29951:

Twenty Fifteen: Swap order of description and caption so the caption is underneath an image on image attachment pages.

Props iamtakashi, fixes #30020.