Auto associate shibboleth account with local account that has the same email

[daronco]

If someone tries to sign in via Shibboleth and the email returned by the federation is an email that already has a local account in Mconf-Web, the user will not be able to sign in. A message is shown telling the user to associate the federated credentials to the local account or to contact an administrator.

It could be simpler and maybe clearer to the user to automatically associate the federated credentials with the local account. With this we would be assuming that the users are the same, but this is very likely always the case since the users have the same email. We would also be assuming that the data in the federation is trusted, even more than the data in the database, which is also a fair assumption.