Add new read link to each entry. If entry was read the link will be
replaced with an unread link.
Clicking the link will mark the entry as read or unread.
Afterwards a redirect to referrer happens. If it is unkown, for whatever
reason, fallback to start.
Flash messages are added and rendered to let user know what happened.
Also respect already read entries when opening a feed.
WIP:
Still need to adjust entry listing for buckets.
Still need to add tests for new feature.
Still need to add new start page to show newest unread entries from all
buckets.
