Abstract email service provider to support others

[stephenyeargin]

Campaign Monitor is great, but some folks may be using other providers like MailChimp. The subscription tab and listeners could be re-worked to support those use cases.

[stephenyeargin]

As this is a core feature of the application, it would need to be well thought out. Each provider will have a different interpretation of how it manages lists and campaigns.

Some areas to ensure are covered:

• Custom field mapping/management (will likely differ among them)
• Listing/linking to subscriber history and campaigns
• Event listener that updates a record on save
• The ESP sync cron job, bin/console app:esp:sync
• The ESP audit task bin/console app:esp:audit

One option would be to migrate what we have to being namespaced/configured as Campaign Monitor, then add yet another one that's more generic (may not have full feature parity at release).