Plugin fixes (#562)

* Plugins: Remove support for V1 manifests

* Plugins: Make proxy endpoints not leak sensitive HTTP headers

* Security: Fix do not forward login cookie in outgoing requests

pkg/api/pluginproxy/ds_proxy.go

@@ -224,7 +224,7 @@ func (proxy *DataSourceProxy) director(req *http.Request) {
 
 	applyUserHeader(proxy.cfg.SendUserHeader, req, proxy.ctx.SignedInUser)
 
-	proxyutil.ClearCookieHeader(req, proxy.ds.AllowedCookies())
+	proxyutil.ClearCookieHeader(req, proxy.ds.AllowedCookies(), []string{proxy.cfg.LoginCookieName})
 	req.Header.Set("User-Agent", fmt.Sprintf("Grafana/%s", setting.BuildVersion))
 
 	jsonData := make(map[string]interface{})