New event modal should adjust options depending of target(s)

This was detected while trying to reproduce some problems @ MDL-67300.

1. Given you are a teacher in a course (A) and member of a group in the course.
2. Given you are a student in another course (B).
3. Given you go to course B calendar*
4. Given you click the New Event button.
5. The option to create event and group events in course B are available, and they shouldn't.
6. If you try to save the course event an "ugly" (nopermissiontoupdatecalendar) error is shown.
7. Instead:
   1. you shouldn't be able to create that event. Modal UI should dynamically allow only correct combinations.
   2. in case previous point is impossible to achieve, the error shouldn't be "ugly" and, instead, show a proper message to the user explaining why that combination is not allowed.

That's it. Ciao

* Both if you go to the calendar from your dashboard and then pick course B. Or if you got to the calendar from course B main page.