prototype: User can create a new challenge. Please check the NewChallengeForm, especially about that 'None'
only authenticated users can create a new challenge
change of the title length
user can add/edit a challenge, based on the same form
more complete form validation for required fields
change url to access to a specific category
add flash messages
add flash messages
add edit link in challenge template (now manage template is useless so i
delete it)
Merge branch 'manage_challenge'