Reduce size of gallery caption

[JustinyAhin]

Fixes #238

Summary

Reduce size of gallery caption.

Relevant technical choices:

No

Test instructions

This PR can be tested by following these steps:

1. Add a gallery block to a post or page
2. Add a caption to the image
3. Select one of the link options in the block sidebar
4. Set the gallery so that the images are fairly small
5. For galleries that have three column and more the font size of the caption is now defined to --global--font-size-xxs which value is 0.75rem.

Quality assurance

• I have thought about any security implications this code might add.
• I have checked that this code doesn't impact performance (greatly).
• I have tested this code to the best of my abilities
• I have checked that this code does not affect the accessibility negatively