[web] Add spoiler style to MarkdownSpoiler

Summary:
We add a unique spoiler style (the same hex code as approved in D5373) to the MarkdownSpoiler component.

https://linear.app/comm/issue/ENG-2191/add-spoiler-style-to-markdowncss

Depends on D5593

Test Plan:
The spoiler text enclosed in || ... || characters are correctly rendered with the new spoiler style visually and in the inspect element console.

Reviewers: atul, ginsu

Reviewed By: atul, ginsu

Subscribers: ashoat, tomek, abosh

Differential Revision: https://phab.comm.dev/D5599