Allow enrollment to multiple groups

Sometimes enrollment to multiple groups is desired. The following changes are applied to the enrollment course element:

• Add new config option "multiGroupEnrollment" with a default value of 1
• Update migrateConfig to add new config option with value 1 for existing nodes
• New UI elements: flag to enable multi-groups (triggered by >1) and input field for number of groups (validates for number >0). No checks for upper limit
• In Run: add verbose infos about multi-groups (upper limit is now configured limit or number of groups from datamodel at runtime)
• In Run: show enroll links if more enrollment is possible

While working in the editor the group and area chooser should be streamlined to match the new style introduced in the contact form.