Fixed CKEditor stuff:

- Added missing insertpre plugin and allow <pre> tag with class
  attribute. Added font style to base.css.
- Removed unused link to old ckeditor.css (now managed by
  django-ckeditor).
Fixed participant csv import for group id

- Allowed to add multiple groups in csv group id field (e.g. "3,4")
- Fixed bug that group ids > 9 can not be imported.
- Updated error message if group id does not exists.
Show supporters on motion slide if available.