CRM-19690 - Enable FlexMailer (if present)

CRM-19690 - Enable FlexMailer (if present)

FlexMailer (https://github.com/civicrm/org.civicrm.flexmailer/) is a
refactoring of the email-delivery logic from the Mailing BAOs.  The primary
goal is to make the email-delivery logic more extensible by exposing a
better set of events for extension-authors.  Sadly, the original code is a
bit toxic (originally lacking in tests; thick with many features; using some
quirky dataflows), which means:

 1. Any refactoring of it poses a high risk.
 2. The refactoring should ideally be done with iteration/validation as
    an optional extension.

This patch aims to be the bare-minimum core patch required to facilitate a
better 'leap by extension'.  The main priorities are:

 1. Minimize risk -- no impact on existing users who can continue using existing logic.
 2. Enable iteration/testing/deployment of an optional extension in real-world scenarios.
 3. Keep any core hacks clear and isolated - don't rashly commit to new, public APIs.