[REF] Start transitioning EmailCommon class to a trait

[eileenmcnaughton]

Overview

This change does not change behaviour but starts the process of using a trait to cleanup the EmailCommon class (which is currently awful)

Before

Form passed to static functions

After

Starting to use a trait

Technical Details

This is a really awful class - I've added a trait & moved the calls from the various forms to that trait
so the code on EmailCommon for buildForm is now only called once. I will follow up with more the same

Comments

[civibot]

(Standard links)

• If this is your first pull-request for CiviCRM, please browse CONTRIBUTING.md for information about the development and testing processes.
• If you are reviewing this pull-request, you may wish to consult the test sites and the Review Standards (long template, short template).

[colemanw]

Nice cleanup.