Implement the select2 widget to the events enrollments

Problem/Motivation

There is no problem, but we are currently using the select2 widget for social_tagging, which offers a user-friendly way of selecting and deselecting items from a select list. For events enrollments we are still using the old format, where it can be hard to delete people once you added them.

Steps to reproduce

- Use the social_tagging to see the new style
- Use the events enrollments to add members to see the old format

Proposed resolution

Implement the select2 widget more broad, starting with events.