Standardise the contact name used in 'To: ' header of emails

CiviCRM currently uses lots of different formats for the contact name when creating an email and using the 'To: ' heading.

This varies between the UI and what is actually sent; and may not be consistent between the two.

My recommendation would be to use 'display name' consistently in the 'To:' header.

Places:

When creating an email to a contact (clicking on their email address on their contact record, and other places)

In \CRM_Contact_Form_Task_EmailCommon::buildQuickForm - sort name is used, which is useful for finding contacts, but not consistent in the display.

When an individual email to a contact is sent (when sending from the above form)

No name is used in the 'To:' header. Only the email address. This is being set here:

https://github.com/civicrm/civicrm-core/blob/4fe5b084839dd66b419287f67b6dd99fb67b9bc2/CRM/Contact/Form/Task/EmailCommon.php#L430

And ending up here:

https://github.com/civicrm/civicrm-core/blob/4fe5b084839dd66b419287f67b6dd99fb67b9bc2/CRM/Activity/BAO/Activity.php#L1475-L1477

When a mailing is sent through CiviMail

The display_name is used, and comes out in the headers.