Add a way to customize html emails via a template and css. Not covered is a way to customize anything not already covered by translation overrides (Admin > Customize > Email Templates).
Some outstanding items:
- [x] Need to test translation overrides for some emails like “user_replied”. (It works)
- [x] Trying to customize html template of all emails might clash with summary emails because there’s so much styling there, and it’s hard-coded instead of applied from Email::Styles.