Move images and PDFs into folders with posts (#235)

This reorganises all the files so that images, PDFs and posts now live in the same folder, rather than separately.

Where before we had this:

```
/app/posts/service-name/YYYY/MM/post-name.md
/app/images/service-name/YYYY/MM/post-name/image-name.png
/app/pdfs/service-name/YYYY/MM/post-name/file-name.pdf
```

we now have this:

```
/app/service-name/YYYY/MM/post-name/index.md
/app/service-name/YYYY/MM/post-name/image-name.png
/app/service-name/YYYY/MM/post-name/file-name.pdf
```

The main advantages are:

* no need to make sure that the folder name in `app/images` exactly matches the post name, which is a common source of mistakes
* easier to create posts using the GitHub web interface, as currently there’s no way to create folders in which to upload images using the web interface without resorting to workarounds
* the Markdown previews in the GitHub web interface now show the embedded images

Author: frankieroberto