Bug when displaying list of users in the Role edition form

[ntarocco]

How to reproduce

Create a new instance of Invenio, with a few users and roles.
Go to admin -> User management -> Roles -> edit one role.
The Users field does not display the select box with the users list.

There is an error in the browser console, see attached screenshot Uncaught Error: No select2/compat/initSelection. It might an issue in Flask-Admin or a wrong bundle/webpack condif. Maybe missing select2 plugin.