Fix the 'Norbert Bug'.

Fix the 'Norbert Bug'.

This bug had two sources:
 - The first source is the fact that we passed a string instead of a list
   as the members argument of mass_subscribe

 - The second source is the fact that python is a stupid languages and
   iterating on a string works, it just enumerates the string character
   per character.

Our founders attributed themselves one-letter email aliases(like 'n' for
Norbert L. and 'y' for Yann B.) and as soon as a list created through the
validation process contained an email with a 'n' (this happen quite often
when the email contains polytechnique), the enumeration of the string
triggered the subscription of 'n@polytechnique.org'.

Signed-off-by: Florent Bruneau <florent.bruneau@polytechnique.org>