Replace quickgrading feedback textbox in mod_assign with an atto box without the toolbar.

Requesting this as currently the textbox doesn't support html formatting and the content for this box comes both from this textbox via quick grading or via an Atto box when slow-grading.

This causes circumstances where hyperlinks, or even standard atto formatting are displayed as tags within a users feedback box.

I think the small textbox when editing grades in the gradebook could benefit from this too.