We'll replace this primarily with per-realm quotas (plus the simple
per-file limit of settings.MAX_FILE_UPLOAD_SIZE, 25 MiB by default).
We do want per-user quotas too, but they'll need some more management
apparatus around them so an admin has a practical way to set them
differently for different users. And the error handling in this
existing code is rather confused. Just clear this feature out
entirely for now; then we'll build the per-realm version more cleanly,
and then we can later add back per-realm quotas modelled after that.
The migration to actually remove the field is in a subsequent commit.
Based in part on work by Vishnu Ks (hackerkid).
Tweaked by tabbott to move changes from the next commit that are
required for this to pass tests into this commit.
Note that this exports a few items that were not previously exported.