Fix dashboard ignored system setting cache (#21621) (#21759)

backport #21621

This is a performance regression from #18058

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Andrew Thornton <art27@cantab.net>

models/system/setting_key.go

@@ -9,3 +9,8 @@ const (
 	KeyPictureDisableGravatar       = "picture.disable_gravatar"
 	KeyPictureEnableFederatedAvatar = "picture.enable_federated_avatar"
 )
+
+// genSettingCacheKey returns the cache key for some configuration
+func genSettingCacheKey(key string) string {
+	return "system.setting." + key
+}