MM-43733: Set concurrency limits via configuration (#20413)

```release-note
We create a new config option MaxImageDecoderConcurrency which
indicates how many images can be decoded concurrently at once.

The default is -1 which means number of CPUs present.

This affects the total memory consumption of the server. The
maximum memory of a single image is dictated by MaxImageResolution * 24 bytes.
Therefore, a good rule of thumb to follow is that MaxImageResolution
* MaxImageDecoderConcurrency * 24 should be less then the allocated memory for
image decoding.
```

https://mattermost.atlassian.net/browse/MM-43733

app/channels.go

@@ -197,8 +197,12 @@ func NewChannels(s *Server, services map[ServiceKey]interface{}) (*Channels, err
 	}
 
 	var imgErr error
+	decoderConcurrency := int(*ch.cfgSvc.Config().FileSettings.MaxImageDecoderConcurrency)
+	if decoderConcurrency == -1 {
+		decoderConcurrency = runtime.NumCPU()
+	}
 	ch.imgDecoder, imgErr = imaging.NewDecoder(imaging.DecoderOptions{
-		ConcurrencyLevel: runtime.NumCPU(),
+		ConcurrencyLevel: decoderConcurrency,
 	})
 	if imgErr != nil {
 		return nil, errors.Wrap(imgErr, "failed to create image decoder")