Custom SMTP servers

[jace]

Hasmail currently sends with a globally configured mail server, which in production happens to be a Sendgrid account that is configured to only send for hasgeek.com. This prevents anyone else from using our hosted instance.

Hasmail should support per-user SMTP settings to send with a user's preferred SMTP server. This can be accepted in one of two ways:

1. Provide a form for custom SMTP settings and accept server name, port, protocol, login and password, and save these against the User model.
2. Provide OAuth-based access for Gmail, as the 2-factor auth makes the password UI cumbersome.

Pending questions:

1. Should users still have the option to send with the globally defined mail server? This may be required when mailing larger databases as Gmail has a 500 email per day limit. If this option exists, how is access to it controlled?
2. Should users be able to specify more than one SMTP server and choose which to send with for each campaign?
3. Should there be an option to share SMTP server settings with other users, by having them owned by an Organization or Team? This option in combination with item 2 above solves the problem posed by item 1.