i18n: Remove quote syntax from stream description notification.

Translators found it confusing, since it's not at all obvious that the
word "quote" should not be translated.

I'm not altogether happy with the code formatting for this.

While we're changing this, also standardize on the "```` quote" style
of quote blocks to ensure code/quote blocks in stream descriptions are
unlikely to conflict with this syntax.

zerver/lib/actions.py

@@ -5353,21 +5353,16 @@ def send_change_stream_description_notification(
     user_mention = silent_mention_syntax_for_user(acting_user)
 
     with override_language(stream.realm.default_language):
-        notification_string = _(
-            "{user} changed the description for this stream.\n\n"
-            "* **Old description:**\n"
-            "``` quote\n"
-            "{old_description}\n"
-            "```\n"
-            "* **New description:**\n"
-            "``` quote\n"
-            "{new_description}\n"
-            "```"
-        )
-        notification_string = notification_string.format(
-            user=user_mention,
-            old_description=old_description,
-            new_description=new_description,
+        notification_string = (
+            _("{user} changed the description for this stream.").format(user=user_mention)
+            + "\n\n* **"
+            + _("Old description")
+            + ":**"
+            + f"\n```` quote\n{old_description}\n````\n"
+            + "* **"
+            + _("New description")
+            + ":**"
+            + f"\n```` quote\n{new_description}\n````"
         )
 
         internal_send_stream_message(