Standardize image captions and fix gallery formatting

[cjyabraham]

On Project Journey Reports there are a few problems:

• captions of images in a gallery don't match the style of the images outside of a gallery; I think we should standardize around those used for standalone images or something similar
• images in a gallery block don't sit side-by-side in columns, they don't honor the columns setting in the editor. Note this also appears to be an issue in case studies and perhaps other content types.

[cjyabraham]

Looks fixed by https://github.com/cncf/cncf.io/pull/568

[cjyabraham]

deployed