Updater progress bar should account for large template updates

[eloquence]

During the fedora-30 to fedora-31 update (from RPM 0.3.0rc1 to 0.3.0rc2), the graphical updater stalls at 95% for about 20 minutes or so. If that time is spent downloading/installing the template, we should consider iterating on the updater behavior to arrive at a more realistic progress estimate.

(I'm labeling this as a bug because the estimate is so far off that it can't really be considered a plausible measure of progress at that point, and can look to the user as if the updater has stalled out.)

[zenmonkeykstop]

We still need a plan for how to swap out old templates for new versions, which will probably not be addressed( by upstream Qubes updater changes, but which will be informed by them. So it's blocked until that work is done.