Add a few cosmetic UI/UX changes

Changes made:

Application.js: 
- The window now scrolls down on appearance of new elements in a survey.

textareaAutoExpand:
- Fixed buggy behavior when the textarea is larger than the window. If the textarea was larger than the window, with the previous behavior, it jumped to the top of the page every time you entered text. 

Response Controller:
- Create instance variables @group and @form
- Add hooks to associate a user with a response (or none)

Response Show View:
- Back button is now styled like the rest of the site, and broken behavior (going to your home page) was fixed. Clicking back now brings you to the response edit view, as it should, using the instance variables mentioned previously in the controller.

User.rb:
- add has_many :responses, see above

Response.rb
- add belongs_to :respondent, class_name: 'User', see above

Groups index:
- This is currently the placeholder front page. Added a warning here about potential insecurity.