Reduce utils.Cfg references (#7650)

* app.UpdateConfig method

* test fix

* another test fix

* the config override option as-was is just error prone, remove it for now

* derp

api4/openGraph.go

@@ -23,7 +23,7 @@ func (api *API) InitOpenGraph() {
 }
 
 func getOpenGraphMetadata(c *Context, w http.ResponseWriter, r *http.Request) {
-	if !*utils.Cfg.ServiceSettings.EnableLinkPreviews {
+	if !*c.App.Config().ServiceSettings.EnableLinkPreviews {
 		c.Err = model.NewAppError("getOpenGraphMetadata", "api.post.link_preview_disabled.app_error", nil, "", http.StatusNotImplemented)
 		return
 	}