widgets: Fix bug where a new line right after /todo broke rendering.

When there was no space right after `/todo` but there was content on a
new line, the message would be rendered plainly, not as a todo widget.
This was because we split on only the space character to then check if
the first token was a valid widget.

Now we split on both spaces and newlines to extract the widget name,
irrespective of whether it is followed by a space or a newline. This
results in the message being rendered as a todo widget as expected.

zerver/lib/widget.py

@@ -8,7 +8,7 @@ from zerver.models import Message, SubMessage
 
 def get_widget_data(content: str) -> Tuple[Optional[str], Optional[str]]:
     valid_widget_types = ["poll", "todo"]
-    tokens = content.split(" ")
+    tokens = re.split(r"\s+|\n+", content)
 
     # tokens[0] will always exist
     if tokens[0].startswith("/"):