Move ImageProxy inside Channels (#18656)

* Move ImageProxy inside Channels

```release-note
NONE
```

* Move back httpService

```release-note
NONE
```

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/post_test.go

@@ -469,7 +469,7 @@ func TestImageProxy(t *testing.T) {
 		*cfg.ServiceSettings.SiteURL = "http://mymattermost.com"
 	})
 
-	th.Server.ImageProxy = imageproxy.MakeImageProxy(th.Server, th.Server.HTTPService(), th.Server.Log)
+	th.App.ch.imageProxy = imageproxy.MakeImageProxy(th.Server, th.Server.HTTPService(), th.Server.Log)
 
 	for name, tc := range map[string]struct {
 		ProxyType              string
@@ -686,7 +686,7 @@ func TestCreatePost(t *testing.T) {
 			*cfg.ImageProxySettings.RemoteImageProxyOptions = "foo"
 		})
 
-		th.Server.ImageProxy = imageproxy.MakeImageProxy(th.Server, th.Server.HTTPService(), th.Server.Log)
+		th.App.ch.imageProxy = imageproxy.MakeImageProxy(th.Server, th.Server.HTTPService(), th.Server.Log)
 
 		imageURL := "http://mydomain.com/myimage"
 		proxiedImageURL := "http://mymattermost.com/api/v4/image?url=http%3A%2F%2Fmydomain.com%2Fmyimage"
@@ -841,7 +841,7 @@ func TestPatchPost(t *testing.T) {
 			*cfg.ImageProxySettings.RemoteImageProxyOptions = "foo"
 		})
 
-		th.Server.ImageProxy = imageproxy.MakeImageProxy(th.Server, th.Server.HTTPService(), th.Server.Log)
+		th.App.ch.imageProxy = imageproxy.MakeImageProxy(th.Server, th.Server.HTTPService(), th.Server.Log)
 
 		imageURL := "http://mydomain.com/myimage"
 		proxiedImageURL := "http://mymattermost.com/api/v4/image?url=http%3A%2F%2Fmydomain.com%2Fmyimage"
@@ -1134,7 +1134,7 @@ func TestUpdatePost(t *testing.T) {
 			*cfg.ImageProxySettings.RemoteImageProxyOptions = "foo"
 		})
 
-		th.Server.ImageProxy = imageproxy.MakeImageProxy(th.Server, th.Server.HTTPService(), th.Server.Log)
+		th.App.ch.imageProxy = imageproxy.MakeImageProxy(th.Server, th.Server.HTTPService(), th.Server.Log)
 
 		imageURL := "http://mydomain.com/myimage"
 		proxiedImageURL := "http://mymattermost.com/api/v4/image?url=http%3A%2F%2Fmydomain.com%2Fmyimage"