Merge branch 'feature/brand-logo-in-emails' into 'master'

app/assets/images/mailers/gitlab_footer_logo.gif

app/assets/images/mailers/gitlab_header_logo.gif

app/helpers/emails_helper.rb

app/mailers/emails/pipelines.rb

app/views/layouts/mailer.html.haml

app/views/layouts/mailer.text.haml

app/views/notify/pipeline_failed_email.html.haml

app/views/notify/pipeline_failed_email.text.erb

app/views/notify/pipeline_success_email.html.haml

app/views/notify/pipeline_success_email.text.erb

changelogs/unreleased/feature-brand-logo-in-emails.yml

doc/README.md

doc/customization/branded_page_and_email_header.md

...ustomization/branded_page_and_email_header/appearance.png

...ion/branded_page_and_email_header/custom_brand_header.png

...ion/branded_page_and_email_header/custom_email_header.png

spec/helpers/emails_helper_spec.rb