MDL-79063 course: Use .text-body for 'disabled' elements in move modal

Using the .disabled class for the course module or section that is
being moved does not have sufficient colour contrast against the
background, especially when it is being highlighted.
So instead of using .disabled, just use Bootstrap classes .text-body
and .font-italic which renders the cm/section to be moved in normal
font colour while preserving its italicised look.