markdown: Make backend status message detection logic mimic frontend.

In 93914d8cd8, we intended to change our
markdown processor to add support for multi-line /me messages.
However, we neglected to change the backend processor, resulting in
the change only taking effect for the user sending the message :(.

We fix this by changing the backend processor too.

Fixes #12450.

zerver/models.py

@@ -1637,7 +1637,7 @@ class Message(AbstractMessage):
         """
         Returns True if content and rendered_content are from 'me_message'
         """
-        if content.startswith('/me ') and '\n' not in content:
+        if content.startswith('/me '):
             if rendered_content.startswith('<p>') and rendered_content.endswith('</p>'):
                 return True
         return False

zerver/tests/test_bugdown.py

@@ -958,7 +958,7 @@ class BugdownTest(ZulipTestCase):
             rendered_content,
             '<p>/me writes a second line<br>\nline</p>'
         )
-        self.assertFalse(Message.is_status_message(content, rendered_content))
+        self.assertTrue(Message.is_status_message(content, rendered_content))
 
     def test_alert_words(self) -> None:
         user_profile = self.example_user('othello')