With the implementation of Spoilers on both web and native, we want to prevent the spoiler text from being revealed in MessagePreview on both platforms. This diff handles replacing the text enclosed with spoiler characters with three black squares, as discussed in the parent tasks for Spoilers, ENG-1472. We previously thought about replacing the spoilers with black square emojis.
The idea with this diff is to "intercept" the raw message text before it is parsed for markdown rules, so we can replace the text enclosed within || ... || before the spoiler characters are removed for the message preview.