Add polite drag reminder

[mirmirmirr]

If a user taps/clicks on a course, there should be some sort of indication or message letting the user know they should be dragging the course instead.

[ccheung777]

@mirmirmirr To clarify, is the reminder supposed to appear when the user clicks on a course in the Semester Block? Or when they click on it in the Toolbox? Or both?