Merge branch '18474-missing-images-on-confirmation-email' into 'master'

Fix images in emails

Closes #18474. The confirmation email is actually the only one that uses the `image_tag` helper at the moment, but this will work for all of them:
```shell
$ ag image_tag -G mailer
app/views/layouts/devise_mailer.html.haml
13:              = image_tag('mailers/gitlab_header_logo.png', id: 'logo', alt: 'GitLab Wordmark')
24:                  = image_tag('mailers/gitlab_tanuki_2x.png', alt: 'GitLab Logo')
```

See merge request !4705

CHANGELOG

@@ -28,6 +28,7 @@ v 8.9.0 (unreleased)
   - Add a metric for the number of new Redis connections created by a transaction
   - Fix Error 500 when viewing a blob with binary characters after the 1024-byte mark
   - Redesign navigation for project pages
+  - Fix images in sign-up confirmation email
   - Added shortcut 'y' for copying a files content hash URL #14470
   - Fix groups API to list only user's accessible projects
   - Fix horizontal scrollbar for long commit message.

config/initializers/default_url_options.rb

@@ -9,3 +9,4 @@ unless Gitlab.config.gitlab_on_standard_port?
 end
 
 Rails.application.routes.default_url_options = default_url_options
+ActionMailer::Base.asset_host = Settings.gitlab['base_url']