Option to add users to roles/workshops

[kdmnk]

Now we only have a way to add roles/workshops on a user's page.
With the current policies, eg. the workshop leaders can not add new users to their workshops.

For simplicity, we can add a button and a new page from the user handling page where users can be chosen for the different roles and workshops.

Later, we can split these into their respective pages (eg. add workshop members on the workshop pages: #20).