#438: Add scroll gallery styles

[rpeterman-gp]

Closes #438

• Adds styles for scroll gallery UX.

To Review

• [x] Checkout Branch.
• [x] Run yarn.
• [x] Run yarn dev:start.

...then...

• [x] Continue review of PRX/cms.theworld.org#166.
• [x] Ensure scroll gallery is rendered and functions as expected. (styles may not match WP styles)
• [x] Make sure to try changing the transition type and resaving post for each transition type.
• [x] Make sure to test fade transition in multiple Webkit and Mozilla browsers.
• [x] Save post as Standard presentation format.
• [x] Ensure scroll gallery slides are shown as regular images in the default (Standard) layout.

[brandonhundt]

@rpeterman-gp A few thoughts:

1. I don't love the styling of the caption. Honestly prefer it in Wordpress. Can you blur that whole background much, much more? and maybe tone down on the border-radius? thanks!

2. Ran into an issue with the "stacked" transition (using Brave) see:

https://github.com/PRX/theworld.org/assets/369739/5669af0b-c21d-4665-9a1a-6872f96d25dd

Otherwise, this is working as expected.