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