MM-49395: Increase compression for previews and emojis (#21985)

We also reduce the max size of uploaded emojis to 512KiB.

https://mattermost.atlassian.net/browse/MM-49395

```release-note
Max size of uploaded emojis is now reduced to 512KiB
to reduce image download bandwidth.
```

app/users/profile_picture.go

@@ -160,7 +160,10 @@ func createProfileImage(username string, userID string, initialFont string) ([]b
 
 	buf := new(bytes.Buffer)
 
-	if imgErr := png.Encode(buf, dstImg); imgErr != nil {
+	enc := png.Encoder{
+		CompressionLevel: png.BestCompression,
+	}
+	if imgErr := enc.Encode(buf, dstImg); imgErr != nil {
 		return nil, ImageEncodingError
 	}