Notify the user of possible clashes when he adds a course

**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**

Find the course-add and slot-selection flow, then trace where clashes are currently removed after adding a course. Verify that a non-obtrusive warning appears as soon as a clash can be detected, including during slot selection when possible.

Written by the indexing model from the issue text.