[FIX] mass_mailing: filter emails to avoid duplicates

When subscribing a user to a mailing list, the `mail.mass_mailing.contact`
created is processed to identify it's name and email from the address (method
`get_name_email` called in `add_to_list` and `name_create`).
For a better consistency, the search of existing contacts should also be done
using the method `get_name_email`.

This avoids that subscribing twice `Example <example@example.com>` fails to
detect duplicates and creates two subscriptions.

Closes #12265

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