Fix un-editable posts when scraping HTML.

Probably also fixed by going to the Settings tab.
Add thread tag "sadev-bugs".
Reload posts when thread's seenPosts changes.

This looked really bad when you had a thread open on iPad, then
swiped-to-delete on the sidebar to clear the unread count. Now the posts nicely
turn unread.

As a bonus, no need to reload posts in -markPostsAsBeenSeenUpToPost: anymore.
Remove useless markingPostsAsBeenSeen property.

This was used to avoid reloading all of the posts in the posts view just
because we were marking some of them read, which would trigger because of the
old beenSeen property on each Post. No more Post property, no need to jump
through these hoops.