samjam: adds 2 more url handlers for:

  * /webcit/postpart/N/name.jpg
  * /webcit/postpart_download/N/name.jpg

where N is a number from 0 upwards counting the attachments to the currently composing post.

It downloads that attachment, and will be used for wysiwyg inline image composing.

If we ever support deleting attachments before post (I guess we will) then struct wc_attachment will need an id number.