Limit feature of optional file of one zite in sidebar.

**Is your feature request related to a problem? Please describe.**

When I help to distribute the optional file of Millchan in one vps with 10 GB free space and I donot know how many optional files would be downloaded into my VPS (the default vaule for big file is 1MB), hours later, I found more than 8 GB optional files of Millchan were downloade into my VPS and they were increasing, I have to delete this zite because this zite would use up all of my free space. I just wanted to help download some new optional file of this zite and the total size of optional flie of this zite should be limited into 1 GB. However, when I click the help to distribute button, more than 8 GB optional files were downloaded into my vps.

**Describe the solution you'd like**
I wish there is a choice in the sidebar that would limit the maximum size of optional files of that zite.

No files, tests, or entry points are named. Start by locating the sidebar's “help to distribute” control and the optional-file download path, then determine how downloads are tracked per zite. Done means a sidebar choice caps the total optional-file size for that zite and prevents further downloads after the cap.

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