Add the new body classes

This is the first step towards implementing the final outcome from the massive discussion on #1697. It adds all the new body classes we need in order to make them match the 'proposed' column in the [context table](https://github.com/TryGhost/Ghost/wiki/Context-aware-Filters-and-Helpers).  The table shows the full details of what would be output in each case.

It will require #2597 to be completed in the future to remove the old classes after we've given theme developers a chance to update. 

The changes are as follows.

- adds `paged` to eventually replace `archive-template`
- adds `page-template `to eventually replace `page` (and `post-template` on pages)
- changes custom page template class to be `page-slug` instead of `page-template-slug`

@JohnONolan I have one remaining concern with this, which is that a post and a page will not share a body class at all posts get `post-template`, and pages get `page-template`. Is this definitely ok?