Page Thumbnail Candidates

Turn Page#thumbnail, which hard coded page type names, into generic
Page#thumbnail_file/Page#thumbnail_url which are resolved from a list
of thumbnail candidates defined in the page type.

Allow new file types to be provide thumbnail css rules in entry
specific css file.
Use Thumbnail Candidates in Editor

Resolve page thumbnails just like in the server side case.
Extract Paperclip Styles for Thumbnails into Config

Allow reuse and modification of generated thumbnails.
Merge pull request #138 from tf/page-thumbnails

Page Thumbnails