Add header spv

I changed the TemplatePicker so that it will never return a photo backdrop. I guess this solution is okay. It might be better to actually remove all the backdrop/wallpaper stuff, but I guess it might be needed again in the future.

I also added a bit of hacky css to make the header render (nearly) the same in the SPV. If you know of a better way, please tell me.

Also, I fixed a bug where the Post would be preloaded in the HTML request with gon, but not actually in a way that was accessible to the Post model in Backbone. It now only needs the one request for the main Post model.