Cohort enrolment - allow multiple cohort selection

This has been mentioned previously in MDL-49417

Adding multiple cohort enrolments to a single course is time consuming. I think it would be better if the drop-list allowed multiple cohorts to be selected at a time, then create an enrolment instance for each cohort selected. (deleting/updating would still need to happen one at a time.) I think this makes more sense than adding support for a single enrolment instance to support multiple cohorts.

To achieve this I think we need to:

1. Remove the ability to set a custom name in the cohort enrolment method
2. make the name show as something like "Cohort enrolment: XXX" on the listing page.
3. modify the cohort selector to allow multiple cohorts to be selected - which would then create an enrolment instance for each cohort.

I think 1/2 will also improve overall UI - at the moment if a teacher sets some basic text in the custom name there is no way on the main list to see what cohort is set in a particular enrolment instance.