Add fallback style for legacy figure captions in the hero

[nicoleslaw]

Expected Behavior

Figure captions display below hero images

Actual Behavior

Figure captions display over the image, and the text is obscured by the image

Steps to reproduce the behavior

• Visit https://18f.gsa.gov/2016/11/17/leadership-innovation-california-child-welfare-services/
• Or https://18f.gsa.gov/2016/10/14/iterative-workplace-design-denver-federal-center/
• Or https://18f.gsa.gov/2016/10/11/getting-the-most-juice-out-of-the-squeeze/
• Check out the hero images

This issue is done when

• @beechnut says a pull request can be opened
• A fall back style is added via PR
• Fig captions display correctly on legacy blog articles and new ones alike

[beechnut]

@nicoleslaw Would you add this to #3864 (post-migration tasks)?

[nateborr]

@nicoleslaw re: the workflow:blocked label on this issue, was that only relevant to the replatforming effort or is anything still blocking work?

[nicoleslaw]

Yes, that was for the replatforming work, thank you!