Fix mail format missing and password min length for user signup app/controllers/user.rb

Corrected display for case where no stats available, put "-" and size blue zone 0% app/views/survey_display.erb

Corrected the various case of blank field in the survey form to avoid bullshit data  app/controllers/index.rb
Correction of the display choice answer list: a user was able to see surveys he has done already.
Change the answer route and the participation/id route.