Add trusted role for a User

This endpoint allows the client to add the trusted role to a user. The user associated with the API key must have an 'admin' or 'moderator' role. The specified user will be assigned the 'trusted' role. Adding the trusted role to a user allows them to upvote and downvote posts and flag content that needs investigating by admins. Users are notified of this change in the UI, and by email.