This will allow easier customization of font family instead of going
through all occurrences of 'mastodon-font-sans-serif'.

* Use PNG images in HTML e-mails

* Make webpack use URLs with host so fonts load inside HTML e-mails

Convert this back to a relative URL in the premailer CSS loader
since local requests are quicker

* Improve responsive design

* Add missing PNG icon

* Remove onboarding modal

* Welcome e-mail

* Send welcome e-mail after confirmation

* Remove obsolete translations

* HTML e-mails for NotificationMailer (except digest)

* Add HTML template for digest

* Fix build

- premailer gem to turn CSS into inline styles automatically
- rework UserMailer templates
- reword UserMailer templates