Association Mail Domain to Structure

Goal, simplification in user association to a structure

1. New administration page to manage mail domain_sub-domain to a structure
   1. in UI for structure administration add a field for mail domain_sub-domain
   2. it's possible to have multiple mail domain for a structure
2. on user creation process, if there is one structure for its mail domain put them directly to the structure
3. when a user choose a structure (in his profil) which is not associated with his mail domain, he must be validated by structure manager
   1. generated a notification to the structure administrator (the message can be edit by the user)