[markdown] Automatic lists should not add additional completed tasks

What does this MR do and why?

When adding a new task list item to a list, if the current task item is marked done, [x], then the new one is automatically marked done.

- [x] a completed task
- [x] 

Instead, always create new, uncompleted task

- [x] a completed task
- [ ] 

Also, recognize [X] as a valid task item, like other task handling does.

Related to #357318 (closed)

How to set up and validate locally

From the rails c, enable the feature: Feature.enable(:markdown_continue_lists)

In an issue comment/description, type in

- [x] item completed

and hit Enter. You should get a new, uncompleted task item.

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