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