Add course exclusivity rules

Add exclusivity groups to course editions. If there are two course editions with the same exclusivity group, then a student can only enrol in one of them.

An explanatory message is shown and the button will be disabled for other courses where they try to enrol.

Adding exclusivity groups can be done by typing a new group name, or by selecting one of the existing groups from the suggestion list. Exclusivity groups are automatically deleted if there are no course editions in them.

Closes #80 (closed)