Configuration option to limit displaying of users in course element members to certain groups

Currently, the member course element displays course members in a nicely formatted way. In some cases however it would be desired to not list all course members but only the one of one or multiple groups.

The goal of this issue is to implement a new configuration option to optionally select multiple groups or group areas to be listed in the members course element.

During runtime, the users are not sorted by group and the group information is not displayed.

The email workflow must be adapted as well to offer only users from the configured group.

The implementation GUI-wise is similar to the one in the mail contact form course element