Make chunk uploads the default for Proguard files

[szokeasaurusrex]

Chunk uploads are available for Proguard files, but they are not the default and have been labelled as experimental. Before we can make them the default, we must do the following:

• [ ] Check whether the server supports Proguard chunked uploads, and only use chunk uploading if yes
• [ ] Investigate and resolve the large memory usage of chunked uploads (#2291)