message_link: Add support for empty string topic in syntax.

This commit adds support for empty string as a valid topic name
in syntax for linking to channel messages.

The server stores it after empty string is replaced with
`realm_empty_topic_display_name` and wrapped with an <em> tag.

The web client parses the rendered_content and updates
the topic_name part in the HTML with topic_name in user's language
+ wraps it in a <span> tag with 'empty-topic-display' css class.

web/third/marked/lib/marked.cjs

@@ -545,7 +545,7 @@ inline.zulip = merge({}, inline.breaks, {
   unicodeemoji: possible_emoji_regex,
   usermention: /^@(_?)(?:\*\*([^\*]+)\*\*)/, // Match potentially multi-word string between @** **
   groupmention: /^@(_?)(?:\*([^\*]+)\*)/, // Match multi-word string between @* *
-  stream_topic_message: /^#\*\*([^\*>]+)>([^\*]+)@(\d+)\*\*/,
+  stream_topic_message: /^#\*\*([^\*>]+)>([^\*]*)@(\d+)\*\*/,
   stream_topic: /^#\*\*([^\*>]+)>([^\*]*)\*\*/,
   stream: /^#\*\*([^\*]+)\*\*/,
   tex: /^(\$\$([^\n_$](\\\$|[^\n$])*)\$\$(?!\$))\B/,