Provides a way for users to actually delte their user drafts where
required.
For #3927
Added test to cover new endpoint.
Makes update to MD editor #setText so that new selection is within new
range, otherwise it errors and fails operation.
This changes how the editors interact with the parent page-editor
compontent, which handles auto-saving.
Instead of blasting the full editor content upon any change to that
parent compontent, the editors just alert of a change, without the
content. The parent compontent then requests the editor content from the
editor component when it needs that data for an autosave.
For #3981
