Spike: Add suppression list support

[mogmarsh]

Description

Users should be able to specify a list or lists of users to not send email to.

Use Case

As an editor, I want to specify a list of emails to not send email to so that I can respect their opt-out preferences.

Acceptance Criteria

• Define a method for specifying a suppression list, which could be provider-specific (use an existing suppression list defined within the provider and accessed via the adapter) or defined within plugin settings or both.
• Create follow-up issues to add support for suppression lists to existing adapters.

[attackant]

@mogmarsh I can jump into this in the new sprint, if you want to give me your two cents on where to begin.

[kevinfodness]

@mogmarsh Can you review the AC bullet above and confirm that's what you had in mind? Feel free to edit as necessary.