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/rendered_markdown.js

@@ -153,6 +153,19 @@ exports.update_elements = (content) => {
         }
     });
 
+    content.find('div.spoiler-header').each(function () {
+        // If a spoiler block has no header content, it should have a default header
+        // We do this client side to allow for i18n by the client
+        if ($.trim($(this).html()).length === 0) {
+            $(this).append(`<p>${i18n.t('Spoiler')}</p>`);
+        }
+
+        // Add the expand/collapse button to spoiler blocks
+        const toggle_button_html = '<a class="spoiler-button" aria-expanded="false"><span class="spoiler-arrow"></span></a>';
+        $(this).prepend(toggle_button_html);
+    });
+
+
     // Display emoji (including realm emoji) as text if
     // page_params.emojiset is 'text'.
     if (page_params.emojiset === 'text') {