Notify the user of possible clashes when he adds a course #186

Is your feature request related to a problem? Please describe.
Since it can be theoretically detected that a clash occurs when you add course, possibly even when you select slot, it can be frustrating that clashes are only deleted when you are done adding course

Describe the solution you'd like
Display a non obtrusive warning like a notification or toast as early as a clash can be detected

Additional context