closes #450

backup database before upgrade!
upgrade script will take the first post of each topic and set the
`mainPid` property on the topic. then it will remove that pid from the
sorted sets for that topic, this was done to make alternative sorting
work.

added a new sorted set called `tid:<id>:posts:votes` that is used to
sort topic posts by vote count, the original sorted set `tid:<id>:posts`
is used to sort by oldest first or newest first.

the main post is added to the returned posts array on topic load and is
always at the top.
theme changes are minimal just a few new data properties on the posts
and the sorting dropdown.
hopefully didn't miss anything too critical.