fixes student survey link to retrieve correct survey
fixes seeds and views when zero students have responded to a survey

Seeds lacked a survey password.
Survey#Show led to breaking pages because no students had taken the survey. Considered sending alerts or showing empty graphs, but decided instead to more simply tell users to come back after students answer the survey.
stops students from taking a survey if enough people have taken it

Problematic if students stop survey halfway then sign in again, meaning that the count is higher than students who have taken it. Uses @survey.students.count < @survey.size. Fix with a find_or_create_by
allows find or create by with unique usernames that incorporate survey id

appends survey id to username before student is created, then performs a find_or_create_by_username_and_survey_id. Consider making the method for making a username more robust. Realistically, this is very fine.