nice alerts

### Done
* Removed alerts for empty form validation, now displaying error in red at bottom of form.
* Removed alerts for invalid access code, now displaying error in red at bottom of form.
* Refactored empty form validation.

### To do 
* Remains the alert in location validation. I could have done it in this pull request, but the location validation will soon use defers instead of callbacks so the message display will be called in the subscription module instead of in the location validation module. When this will be done, call displayError in subscription and remove alert in the location validation. I wrote 'location validation' 5 times in 4 lines.
I'm not sure but I think you could avoid repeating this alert container in very 'step' by simply move it in the parent div that has the .signup class

you could then have a single container where all the signup error messages are displayed. we probably talked about this before, maybe there was a reason you didn't do it that way ??