Resolve "Button to add User from LDAP in management panel"

Added a button to create a new user:

This opens a new modal, where you can search for users. After users are retrieved, their name, email and affiliation are shown:

A user can be created on ProjectForum by clicking on their name:

Already existing users are not created again. If that is the case, this is silently handled and no error is shown to the user, a new user is not created.

Admins can also click on the "Created" label in order to go directly to the user page.

Closes #377 (closed)