bugdown: Fix $$ whitespace handling.

The intended use of $$ is for inline expressions, not for multiline
ones; ```math is an acceptable alternative for the latter.  Hence,
the $$-syntax for inline TeX no longer permits newlines within it.

This was also necessary for the next change to be sensible; namely
allowing for spaces around both $$ when crafting inline TeX instead of
forcing everything to be crammed together, e.g. $$x=7$$.  In order to
avoid uninentionally creating inline expressions, the opening and
closing $$'s of an inline expression must now both exactly consist of
two dollar signs, no more and no less.

Fixes: #6488.

static/third/marked/lib/marked.js

@@ -519,7 +519,7 @@ inline.zulip = merge({}, inline.breaks, {
   stream: /^#\*\*([^\*]+)\*\*/,
   avatar: /^!avatar\(([^)]+)\)/,
   gravatar: /^!gravatar\(([^)]+)\)/,
-  tex: /^(\$\$([^ _$](\\\$|[^$])*)(?! )\$\$)\B/,
+  tex: /^(\$\$([^\n_$](\\\$|[^\n$])*)\$\$(?!\$))\B/,
   realm_filters: [],
   text: replace(inline.breaks.text)
     ('|', '|(\ud83c[\udd00-\udfff]|\ud83d[\udc00-\ude4f]|' +
@@ -527,6 +527,7 @@ inline.zulip = merge({}, inline.breaks, {
           '[\u2000-\u206F]|[\u2300-\u27BF]|[\u2B00-\u2BFF]|' +
           '[\u3000-\u303F]|[\u3200-\u32FF])|')
     (']|', '#@:]|')
+    ('^[', '^^\\${3,}|^^[')
     ()
 });