Warn if a substitution conflicts with a regular lesson of the substituting teacher
E.g. DOE has a regular lesson on Tuesday, 2. period. Currently, it is possible (without a notice/warning) to select DOE as a teacher for substituting another lesson.
In addition to the warning, an option should be given to continue to the substitution edit page of the conflicting lesson. This is useful in cases where two teachers are planned for the regular lesson and one of them can be moved to another course for substitution.