This adds support for a "spoiler" syntax in Zulip's markdown, which
can be used to hide content that one doesn't want to be immediately
visible without a click.
We use our own spoiler block syntax inspired by Zulip's existing quote
and math block markdown extensions, rather than requiring a token on
every line, as is present in some other markdown spoiler
implementations.
Fixes#5802.
Co-authored-by: Dylan Nugent <dylnuge@gmail.com>
Sara Gulotta