Add skeleton classes for Poll, User, Question, Answer, and Team models
Add Team, User, Poll, Question, Answer, and Response models
Remove validation for team_id from User class
Add Poll#add_question
Add validation for Answer model
Add Question#add_answer method
Add validation for Response model
Fix validation code for Answer and Response classes
Add seed file
Add Question#results
Fix Response#hasnt_already_responded
Add User#poll_list
Add User#response_list
Add validation method Response#on_team
Add dependent actions to Team, User, Poll, Question, and Answer models
Add indexing for user_id column in responses table