Add Category Field to Post Actions, Fix markupsafe Usage

Add and Edit Post now support categories. View Category now supports editing
the active category.

I ran into some problems with wrapping the markdown output with Markup() from
markupsafe. Apparently it keeps the Markup wrapper, which meant that the 'Post'
field on Edit Post was filled with markup. I solved this problem by telling
Mako not to escape anything instead.
Modify Stylesheets

I'm pretty satisfied with the current style of the site. I need to attribute
the color palette still.
Fix Category Submission for New Posts

I forgot to actually handle the data from the dropdown that I added to
/post/new/. That's fixed now, and adding a post to a selected category actually
works.
Trying to get rid of extra files that shouldn't be in the repo