Allow setting up SMTP sender without autentication

Some internal SMTP systems might be configured to not require authentication. Keila should support this by letting users pick the authentication method. This should be done in the following way:

• [ ] Add a new field to the SMTP sender settings: smtp_auth_method. It should support the values password and none. Empty values should also be allowed for backwards-compatibility.
• [ ] Update the to_swoosh_config function so that it does the following: When smtp_auth_method is set to "password", set: auth: :always, if it's set to "none", set: auth: :never. Else, set explicitly set auth: :if_available.
• [ ] Add a dropdown to the SMTP sender config that lets users choose the authentication method. The options should be password and none. password should be the default.