Fix #3961

Bumps [webpacker](https://github.com/rails/webpacker) from 3.5.5 to 4.0.2.
- [Release notes](https://github.com/rails/webpacker/releases)
- [Changelog](https://github.com/rails/webpacker/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rails/webpacker/compare/v3.5.5...v4.0.2)

Signed-off-by: dependabot[bot] <support@dependabot.com>
Co-authored-by: Yamagishi Kazutoshi <ykzts@desire.sh>

* 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

* Change image URL in mailer to full path

* Add application_mailer.view_profile localization.

* 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