[FIX] auth_signup: remove password from welcome message

Revision 054c6868 added by mistake the
password of the new user in the welcome message that is sent to users
who signed up with an invitation token.

There is no need for this, the user has just chosen their password, and
we should not send them a copy which could be compromised on the way to
their inbox or later in their inbox.
It may also give users the impression that their passwords is stored in
cleartext in the database, even when that is not the case.

This patch minimalizes changes to the template and its translations.
For existing databases where the code is updated without re-syncing the
template, the password will simply be missing in the message (until a
resync of auth_signup module is done)