Mimemail sometimes sends emails with internal usernames

Problem/Motivation

When sending emails to a user, and if there's a plaintext field configured, Mimemail can send the username of that user, but it's using the internal username:
$address->getAccountName() rather than the more appropriate: $address->getDisplayName()

Steps to reproduce

Enable a plain-text configuration field, send an email to a user, ideally on a site that customises the display name.

Proposed resolution

Separate the logic for when to send using usernames, and then also change to using getDisplayName()

Remaining tasks

Needs some tests.

User interface changes

Emails received will probably use a username.

API changes

None.

Data model changes

None.