Add template and categories fields to "New Post" form

[tboutell]

Right now the new post form makes you provide a title, which solves a lot of chicken and egg problems with the post not having a slug yet, etc.

Geoff proposed that we add the template selector and category selector fields to this form.

Adding the template selector would address the fact that users (Geoff and Nikola on the list) find it strange that you have to click "update" after picking a template to get the right-hand area with the content slots to refresh.

Adding the category selector helps because it is necessary on most sites to categorize content before it will show up at all on the blog engine page you had in mind and it helps to draw attention to anything that is absolutely necessary.

As a further improvement in this area, if we came to "new post" from an engine page with one (1) category, the category selector would have that one category preselected. In this situation there is no ambiguity- you don't click "new post" on the Cats Blog unless you want to write a post that will show up there, and the only way to get that is to select Cats from the categories list. In any other case no categories would be preselected.

There are deeper solutions here but they open much larger cans of worms and belong in 2.0 tickets. This is a safe and incremental step to improve the usability of the version we'll be supporting for quite a while. When changing 1.5 we have to think small