Add onboarding state selector for self-hosted signup (#251)

* Add onboarding modes to self-hosted signup

* Style form consistently

* Configure ONBOARDING_STATE via ENV

app/controllers/concerns/invitable.rb

@@ -8,7 +8,11 @@ module Invitable
   private
     def invite_code_required?
       return false if @invitation.present?
-      self_hosted? ? Setting.require_invite_for_signup : ENV["REQUIRE_INVITE_CODE"] == "true"
+      if self_hosted?
+        Setting.onboarding_state == "invite_only"
+      else
+        ENV["REQUIRE_INVITE_CODE"] == "true"
+      end
     end
 
     def self_hosted?