Review email configuration

[tcompa]

We have mail_settings.use_starttls, and we should add mail_settings.use_login (defaulting to True).
Ref:

• Add include_login option fractal-healthcheck#23

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Let's also take this opportunity to make the configuration variables more readable. First guess, it should be like this:

1. All FRACTAL_EMAIL_* variables are optional
2. As soon as one of the variables is set, all of them are required.
3. Exceptions:
   • FRACTAL_EMAIL_USE_{STARTTLS,LOGIN} should be optional and default to True
   • FRACTAL_EMAIL_PASSWORD should only be required if FRACTAL_EMAIL_USE_LOGIN is set to True
4. FRACTAL_EMAIL_PASSWORD should still be handled the same way as now (with fernet encryption)