[web] Reveal the spoiler content on click within a message

Summary:
We add an onClick function to allow the spoiler content to be revealed on click. This onClick function then removes the spoiler class and animates the "reveal" effect to display the text content. The spoiler tags will not be re-rendered until the messages are unmounted.

https://linear.app/comm/issue/ENG-2192/reveal-the-spoiler-content-on-click-within-a-message

Depends on D5599

Test Plan:
Spoiler tags are revealed as expected, with a timed animation to fade the background and they get re-rendered after leaving and re-opening the chat.

Reviewers: atul, ginsu, ashoat

Reviewed By: atul, ginsu, ashoat

Subscribers: ashoat, tomek, abosh

Differential Revision: https://phab.comm.dev/D5601