Fix editing, replying, and posting when "Hide Google/Amazon Search Bars?" is set to "no".

We were scraping the wrong form, as I'd naively assumed the first form was
always the right form. Now we're looking for parameters we expect to find,
which seems less fragile in general.
Update thread's last post date/user when scraping last page.
Fix loading pages that are not encoded as win1252.

Sometimes I guess SA's response headers don't lie?
Don't crash when locating posts.

Passing a string where a post was expected is not helpful!