Use appEvents for email sending, so that plugins can integrate with non SMTP email providers

[dancastellon]

Prerequisites

• Are you running the latest version? 2.0 RC5
• Are you able to consistently reproduce the issue?
• Did you search the issue queue for existing issue? Search issues

Issue Description

Currently, Reaction only supports sending email through an SMTP account. This logic is hardcoded in imports/plugins/core/core/server/Reaction/processEmailJobs.js. There is no way to plug in to the email job handling and use a third-party email provider that doesn't use SMTP.

Possible Solution

Use the new appEvents to allow other plugins to process email jobs and send with any email service. Example: appEvents.emit("processEmailJobs") from processEmailJobs.js, and appEvents.on("processEmailJobs", (jobs, callback) => CUSTOM_LOGIC_HERE).