Support Creating New Draft Thread (instead of comment)

Following up on !51718 (comment 505591161)

At the moment the drafts database table doesn't house a field to determine whether a draft should be resolvable or not. It seems implicit that if the draft reference a diff it is resolvable.

We will likely need to extend the table to support this.

Alternatively, we might just be able to hook in the method which converts a draft to a note and force it to always be resolvable (when it's not a reply to an existing discussion).