Connect the existing "Preview" tab to the live-preview functionality

As a transitional state towards the use of the SE toolbar, we want to prime the users to use the new live-previe functionality when editing the markdown field.

At the moment, the functionality is available as a contextual menu in the editor itself, or as a button on the SE toolbar for the users who has the source_editor_toolbar flag enabled. This issue is aiming at unifying the preview experience for markdown editing.