Don't allow to add board member multiplie times in same board + render only limited member information
We currently can add the same user over and over to the same board. Make it so that we get an error when adding a user that is already a board member.
Also, no need to have all the users information in the users array of the /boards
api (index and show). We only need those: id, first_name, last_name, logo_url_sm, logo_url, short_bio.