Task: Add "Convert to JPEG" checkbox to photo uploader

Very often images are uploaded in PNG format, but it makes no sense because these are not interface elements, they are all opaque, and at the same time they take up several times more space. We need to convert them to JPEG right away.

We just need to check the frontend multi-uploader, since hashes are calculated right on the frontend there, so this conversion will need to be done on the frontend as well.

Right now, all this might be unnecessary development overhead, and it will be enough to provide conversion for the frontend in the resizer + Varnish caching.