Remove Flipper and replace with ENV-driven FeatureFlags (#957)

* Presence of valid DEFAULT_UI_LAYOUT is sufficient

* Linter

lib/feature_flags.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+module FeatureFlags
+  class << self
+    def db_sso_providers?
+      auth_source = ENV.fetch("AUTH_PROVIDERS_SOURCE") do
+        Rails.configuration.app_mode.self_hosted? ? "db" : "yaml"
+      end
+
+      auth_source.to_s.downcase == "db"
+    end
+
+    def intro_ui?
+      Rails.configuration.x.ui.default_layout.to_s.in?(%w[intro dashboard])
+    end
+  end
+end