Back to reverse chronological (for now)
Add factor to feeds, to speed up or slow down their decay
Use identical timestamp for updates during initial check

Otherwise the ages can vary a bit depending on when in the randomized
feed list the feed was checked. When in reality, the "clock" is only
really meaningful for new items seen after the initial check.
Display how long until the next feed list check
Don't include size of individual update JSON
Only use Feed.started during initial check

Otherwise adding a feed once in the "running loop" causes it to be
timestamped with when river was started.
Delete any previous JSON file before starting