todo: Require space after colon to separate task title from description.

This fixes the annoying behaviour where a task title with a url in it,
for example, "Fix issue https://github/com/zulip/zulip/issues/1234"
would be wrongly split into a description at the `:` in the url.

help/collaborative-to-do-lists.md

@@ -12,7 +12,7 @@
 
 2. Type `/todo` followed by a space, and the title of the to-do list.
 
-3. _(optional)_ Type each task on a new line, with its description, if any, after a <kbd>:</kbd>.
+3. _(optional)_ Type each task on a new line, with its description, if any, after a <kbd>:</kbd> and blank space.
 
 4. Click the **Send** (<i class="zulip-icon zulip-icon-send"></i>) button, or
    use a [keyboard shortcut](/help/mastering-the-compose-box#toggle-between-ctrl-enter-and-enter-to-send-a-message)

zerver/lib/widget.py

@@ -56,7 +56,7 @@ def parse_todo_extra_data(content: str) -> Any:
         task_data = re.sub(r"(\s*[-*]?\s*)", "", line.strip(), count=1)
         if len(task_data) > 0:
             # a task and its description (optional) are separated
-            # by the (first) `:` character
+            # by the (first) `: ` substring
             task_data_array = task_data.split(": ", 1)
             tasks.append(
                 {