Implement brand image endpoints for APIv4 (#5733)

* Implement brand image endpoints for APIv4

* Fix unit test

api4/brand_test.go

@@ -0,0 +1,69 @@
+// Copyright (c) 2017 Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package api4
+
+import (
+	"net/http"
+	"testing"
+)
+
+func TestGetBrandImage(t *testing.T) {
+	th := Setup().InitBasic().InitSystemAdmin()
+	defer TearDown()
+	Client := th.Client
+
+	data, resp := Client.GetBrandImage()
+	CheckNoError(t, resp)
+
+	if len(data) != 0 {
+		t.Fatal("no image uploaded - should be empty")
+	}
+
+	Client.Logout()
+	data, resp = Client.GetBrandImage()
+	CheckNoError(t, resp)
+
+	if len(data) != 0 {
+		t.Fatal("no image uploaded - should be empty")
+	}
+
+	data, resp = th.SystemAdminClient.GetBrandImage()
+	CheckNoError(t, resp)
+
+	if len(data) != 0 {
+		t.Fatal("no image uploaded - should be empty")
+	}
+}
+
+func TestUploadBrandImage(t *testing.T) {
+	th := Setup().InitBasic().InitSystemAdmin()
+	defer TearDown()
+	Client := th.Client
+
+	data, err := readTestFile("test.png")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	ok, resp := Client.UploadBrandImage(data)
+	CheckForbiddenStatus(t, resp)
+	if ok {
+		t.Fatal("Should return false, set brand image not allowed")
+	}
+
+	// status code returns either forbidden or unauthorized
+	// note: forbidden is set as default at Client4.SetProfileImage when request is terminated early by server
+	Client.Logout()
+	_, resp = Client.UploadBrandImage(data)
+	if resp.StatusCode == http.StatusForbidden {
+		CheckForbiddenStatus(t, resp)
+	} else if resp.StatusCode == http.StatusUnauthorized {
+		CheckUnauthorizedStatus(t, resp)
+	} else {
+		t.Fatal("Should have failed either forbidden or unauthorized")
+	}
+
+	_, resp = th.SystemAdminClient.UploadBrandImage(data)
+	CheckNotImplementedStatus(t, resp)
+}