Add marketing list capabilities + integrate with email provider

[matoszz]

Our current transactional email provider (Sendgrid) is today just blasting emails out via API requests - given we have the user context when sending the email, we should start tracking contact attributes via sendgrid's Marketing Campaigns for future marketing / email purposes. The current email provider integration is specific to sendgrid and needs to be eventually genericized and made to be a customer-facing integration capability (or at least highly customizable) but this issue is to start building those features out by enhancing our existing integration to take advantage of SG's platform features.