Improve calendar events for groups (make it clearer teacher can only add those to courses where they is part of a group/s)

Hi,
When a teacher (or admin) tries to create a group event:

• On LMS: the teacher can select any course that they is enrolled in AND is part of a group/s
• On the app: the teacher can select any course that they is enrolled in (independently if they is part of a group/s). if they selects a course where they aren't part of any group, an error message is being shown “You are not a member of any group of this course”
  

IMHO the behaviour on the app should be the same than on the LMS