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 taks for Spoilers, [ENG-1472](https://linear.app/comm/issue/ENG-1472/spoiler-quotes-in-markdown). 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.
[Context Diff](https://phab.comm.dev/D693)
Addresses [ENG-2193](https://linear.app/comm/issue/ENG-2193/prevent-spoiler-text-from-appearing-in-messagepreview-[web])
Addresses [ENG-2055](https://linear.app/comm/issue/ENG-2055/prevent-spoiler-text-from-appearing-in-messagepreview-[native])