Shows the publishing time of the latest post for feeds

[shizunge]

In the feed page:
For each feed, shows the publishing time of the latest post.
This may be conflict with #699, as we could show to many information on a single page.

Allow user to sort feed by the latest post time (#632 )

[fguillot]

Why?

[shizunge]

1. Feed is an aggregation of articles. We show the publishing time of articles. It makes sense that the aggregation of publishing time is also publishing time.

2. I find the latest publishing time is more useful:
   Latest publishing time of a feed could help us to find inactive feeds (if feeds are sorted by the latest publishing time)
   It also allows us to check whether there is an updates of a feed in the feed page, without going into the article list.
   Although it may be useful for developers, I did not find the "last check" time is very useful for end users. But as an end user, I always assume Miniflux fetches articles correctly and I do not care about the latency between when the article is published and when it gets fetched.