Allow todo items inside bulleted lists & other nesting structures

It would be awesome if we could put todo items anywhere as inline items instead of them needing to be top-level block content in a given text container.

One example of this would be allowing a todo item to be created under a nested bullet item in a list.

Steps to reproduce:

1. Create a list (click bulleted list item in Confluence's editor)
2. Type text in the first bullet
3. Hit `Enter` to create a second list item
4. Hit `Tab` to nest the second list item under the first
5. Click the new todo item button in the editor menu

Current behavior:
A todo item is placed after the list entirely, outside of the bulleted items.

Desired behavior:
A todo item is placed within the nested bullet item, as an inline element comprised of a checkbox, text and optional mentions.