Debug email subject settings

[joemull]

Closes #2313.

The following email subject settings were broken and had no effect. Now they will correctly populate outgoing emails.

• subject_unassign_editor
• subject_review_complete_acknowledgement
• subject_review_decision_accept
• subject_review_decision_decline
• subject_copyediting_decision
• subject_author_publication
• subject_section_editor_pub_notification
• subject_peer_reviewer_pub_notification
• subject_revisions_complete_receipt

Editors are encouraged to check any of these settings that have been modified in the past.