Download .sb3 files with a custom mime type

[paulkaplan]

We should use a custom mime type, instead of application/zip, to download files from the browser, so that they are easier to open on platforms that use mime types instead of file extensions (i.e. iOS/Android).

Before doing this, we need to check with scratch-desktop to see if anything there would have to change in terms of opening files of both mime types (since both will be floating around in the wild). /cc @cwillisf I'll bring this up with you offline