Merge branch 'fix-devise-email-display-name' into 'master'

Use the configured display name in e-mails from Devise mailer

By default, it is set to "GitLab", but a user may configure the display name to something else.

See merge request !613

app/mailers/devise_mailer.rb

 class DeviseMailer < Devise::Mailer
-  default from: "GitLab <#{Gitlab.config.gitlab.email_from}>"
+  default from: "#{Gitlab.config.gitlab.email_display_name} <#{Gitlab.config.gitlab.email_from}>"
   default reply_to: Gitlab.config.gitlab.email_reply_to
 end