Add HTML-compliant e-mail defaults and plain text e-mail defaults to Notifications

Current Situation

• The Notifications sub-view by default is populated with Message elements for HTML e-mails that are applied in case of success or failure of jobs and workflows.
• The default HTML e-mails are not standard-compliant and rendering can be denied by e-mail clients.

Desired Behavior

• Defaults for HTML e-mails are provided that are compliant to HTML5 standards.
• In addition, defaults for plain text e-mails are provided.

Maintainer Notes

• An existing configuration for Notifications is not updated should a maintenance/minor release be applied.
• Users who wish to apply the updated e-mail templates find the attached notification.xml file for upload to the Notifications view.