Move plugins under Channels (#18938)

* Move plugins under Channels

We move all plugin related fields under *Channels.
This essentially migrates several methods from being under
*Server to under *Channels.

We also move the plugin startup and shutdown code
to be under Channels.Start and Channels.Shutdown.

While here, we remove the getPluginPublicKeyFiles
method which was a one-line method which uselessly
returned an error.

Lastly, we fix the product initialization order which
was incorrect previously. Products are dependent on
the main server.

So startup should be products -> server.
And shutdown should be server -> products.

```release-note
NONE
```

* Added app layer

```release-note
NONE
```

* Incorporate suggestions

```release-note
NONE
```

app/app.go

@@ -133,6 +133,9 @@ func (a *App) ImageProxy() *imageproxy.ImageProxy {
 func (a *App) Timezones() *timezones.Timezones {
 	return a.ch.srv.timezones
 }
+func (a *App) License() *model.License {
+	return a.Srv().License()
+}
 
 func (a *App) DBHealthCheckWrite() error {
 	currentTime := strconv.FormatInt(time.Now().Unix(), 10)