Add ability to add users while performing a bulk edit, instead of "change" users in the fields.

When performing a bulk action to edit the selected issues, the only available option is to "change" the current users on that field for new ones.

In this case, the custom field type "Multi user" cannot "add" users in the bulk edit option.

The suggestion is to add a possibility to retain the users already stored for each issue while adding more users to the field.