markdown: Add support for spoilers.

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>

static/js/ui_init.js

@@ -438,6 +438,7 @@ exports.initialize_everything = function () {
     subs.initialize();
     stream_list.initialize();
     condense.initialize();
+    spoilers.initialize();
     lightbox.initialize();
     click_handlers.initialize();
     copy_and_paste.initialize();