Extension email template registration

[ctml91]

Feature Request

Emails from core & extensions feel a bit fragmented, because every extension seems to provide their own way to send notifications as emails. Sometimes they are hard coded, sometimes they have custom settings to change their contents with basic text, and sometimes they may support HTML emails. This means some emails to users may include a your sites branding, some may just be plain text, some may have grammatical errors you have no option of changing.

Describe the solution you'd like

It would be great to see either in this extension or core, a way for extensions to register email notifications & their templates so they can all be customized in the same way a subset of common notification email templates are in this extension.

Describe alternatives you've considered

The only other solutions are to rely on developers to try and all align on supporting similar formats (allowing to send HTML, with customized templates per extension in the extensions settings).