show error message if the user attempts to add a visit to a marked subject (deceased, resigned, excluded, etc)

check if there are unfinished visits before allowing the user to create a new visit. This will avoid problems of the user going back on the browser after creating a visit because it will be redirected to the list of visits

address, city, country, data_born, default_written_communication_language, email, first_name, languages, last_name, phone_number, phone_number_2, phone_number_3, postal_code removed from study_subject (all this information is stored in subject)

the code is fixed so unit test passes