Differentiate actual read item and marked-as-read item

[shizunge]

Separated from #668

Today an article is marked as read when

1. we view it (i.e. from /unread/entry/* or /history/entry/*)
2. use the mark-all-read/mark-page-as-read action.

We want to differentiate these behaviors. Thus we could tell which feed we interact most. We call the 1st one viewed, the 2nd one marked.

We need:

1. In the "feed" page: For each feed, shows three numbers (instead of two): unread count, read count and total count. Sort feed by viewed articles. (#632 )

2. Show only viewed items in the history page.