Slice upload of comparison screenshots on HDB.

[GizmoBal]

The upload of comparison screenshots on HDB started to fail 2 weeks ago.
The reason appeared to be that the upload size is now limited to 100MiB.
https://hdbits.org/forums/viewtopic?topicid=81527&page=p1497682#1497682
This limitation seems to be set by CloudFlare and results in a silent failure of any upload whose size is larger than the limit.
The workaround was to slice the upload in smaller chunks.
The submitted PR proposes to make one upload to imgHDB per comparison group.
This has only been tested in debug mode so far, and worked successfully in debug mode.

[Audionut]

I noticed the size limit for #785 but it's not been pushed, and doesn't handle this use case.

I've updated the chunking so as to group sorted_group_indices until it reaches a 100 MiB limit, which should be better overall.
Can you double check please.

[GizmoBal]

Nicely done.
I tested in debug mode on 2 distinct examples that were already uploaded to HDB. I knew for sure that the image upload size exceeded the 100 MB limit, as the image upload failed without the patches in that PR. With the PR, each upload was split into 2 chunks. I checked that the images were successfully uploaded to img.hdbits.org and that the corresponding generated files '[HDB]DESCRIPTION.txt' were correct.
Everything seems to work smoothly, now.
Btw, thank you very much again for this wonderful addition of comparisons into UA. It has become an essential part of my workflow and I am now addicted to it. :-)