FEATURE: pick a valid hostname for notification email based on hostname

FEATURE: allow notification email to be set via ENV

config/site_settings.yml

@@ -31,6 +31,7 @@ required:
   notification_email:
     default: 'noreply@unconfigured.discourse.org'
     type: email
+    shadowed_by_global: true
   site_contact_username:
     default: ''
     type: username

db/fixtures/999_settings.rb

@@ -0,0 +1,4 @@
+if SiteSetting.notification_email == SiteSetting.defaults[:notification_email]
+  # don't crash for invalid hostname, which is possible in dev
+  SiteSetting.notification_email = "noreply@#{Discourse.current_hostname}" rescue nil
+end