Provide feed statistics and update rate suggestions

[tanriol]

https://forums.mozilla.org/addons/viewtopic.php?p=29595

I have plenty, but one thing I wanted Brief, and now Digest, to do was make it a bit easier to manage feeds, by adjusting (or advising me to adjust) the rate at which individual feeds are polled. In the long-term, by tracking the pattern of articles as they come in, it should be possible to predict (not completely of course) how many articles will come in the next few hours, or next day, or next week, or next month. Based on that, it should be able to decide for me how often to poll each feed. I have hundreds of feeds, so it's not practical for me to set it myself on each feed, and a fixed setting doesn't take into account time-of-day or weekly fluctuations in articles. Some sites publish all their articles for the month on a single day. So I shouldn't be polling them the other 29 days, unless they were late posting them.

This is a big feature, so probably should be approached in stages.. the two most effective changes right away would be:

1. try to notice that a feed hasn't been updated in a while, or has been persistently unreachable. It could notify via the feed's icon that perhaps instead of simply not having anything new to read, it may actually be dead. Brief does change the icon briefly to a red exclamation triangle I think, but then it changes back after some period of time, and it's only based on a single failure to read the feed (which could be due to anything) rather than a persistent problem. This feature would probably be useful for all users.
2. try to notice that if each time a particular feed is read, everything is new, then probably the feed should be read more often until it starts detecting some already-known articles in the feed. This feature may not be useful for people who don't keep Firefox running 24/7, as I do, but would help me a lot.