Add /v4/image api (#8230)

* add image api

* i suppose i should add a test...

* only redirect to image proxy

api4/image.go

@@ -0,0 +1,22 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package api4
+
+import (
+	"net/http"
+)
+
+func (api *API) InitImage() {
+	api.BaseRoutes.Image.Handle("", api.ApiSessionRequiredTrustRequester(getImage)).Methods("GET")
+}
+
+func getImage(c *Context, w http.ResponseWriter, r *http.Request) {
+	// Only redirect to our image proxy if one is enabled. Arbitrary redirects are not allowed for
+	// security reasons.
+	if transform := c.App.ImageProxyAdder(); transform != nil {
+		http.Redirect(w, r, transform(r.URL.Query().Get("url")), http.StatusFound)
+	} else {
+		http.NotFound(w, r)
+	}
+}