UX: Add validator for SiteSetting#sso_overrides_email.

lib/validators/sso_overrides_email_validator.rb

@@ -0,0 +1,20 @@
+class SsoOverridesEmailValidator
+  def initialize(opts={})
+    @opts = opts
+  end
+
+  def valid_value?(val)
+    return true if val == 'f'
+    return false if !SiteSetting.enable_sso?
+    return false if SiteSetting.email_editable?
+    true
+  end
+
+  def error_message
+    if !SiteSetting.enable_sso?
+      I18n.t('site_settings.errors.enable_sso_disabled')
+    elsif SiteSetting.email_editable?
+      I18n.t('site_settings.errors.email_editable_enabled')
+    end
+  end
+end