Compress post/thread handlers into a single path

[bvisness]

We have lots of highly redundant code for all the different kinds of posts/threads across the site: forums, subforums, blogs, and even some legacy things that aren't in active use any more. All the handler code for these is duplicated and could easily be condensed.

URLs can remain the same; we still need to be able to distinguish what kind of post you are creating or editing. But all the handlers can probably map to the same common handler code with some flags / properties for what type of post/thread you're creating, what project it's associated with, etc.

Would be useful to take care of this before #14, since now we're postponing that until after the X-Ray Jam.