[IMP] website_mail: improved snippet to subscribe to a mail.group.

Main modifications :
- layout: input - button when not following, links (email - archives - unsubscribe) when following
- when adding your email, update all other subscribe snippets input in the page to avoid havign to re-type it
- management of fields of the record to subscribe to, used to have access to the alias of the group
Merge pull request #213 from odoo-dev/saas-4-group-snippet-imp-tde

[IMP] website_mail: improved snippet to subscribe to a mail.group