Email adapter interface and initial implementations(s)

[molomby]

A framework for managing the generation and handoff of emails and one or more initial/core implementations.

The ability to generate emails and hand them off reliably, ideally in a portable (cross provider) way, are requirements that effect almost all projects. A full solution would include:

• An defined interface that mail gateway packages can support (thematically similar to the data store adapter interfaces)
• One or more gateway implementations against this standard (eg: Postmark, Mailgun, Mandrill, Sendgrid, etc.) and probably a stub gateway for dev/debug that dumps calls to the console or disk
• Ability to provide a mail gateway instance to Keystone for use internally (eg. forgotten password, email address verification, etc.)

@JedWatson points out there's significant prior art for this in KS4. Ideally we could port it directly (less work and backwards compatibility).

My preference would be an adapter for Postmark.

[jesstelford]

Don't forget OneSignal's free platform (which supports SendGrid, Mailgun, and Mandrill with more coming soon

[jesstelford]

We now have the @keystone-alpha/email package (which wraps the keystone-email package) to help with this.