Added thumbnail image for each post

Added a thumbnail column to Post object to hold string for post thumbnail img url.
Added a new function called render_thumbnail to generate the thumbnail on index if it exists for that post.
Added an upload dialog on edit.html so a thumbnail image can be uploaded for each post.
Added some styles to handle the thumbnail image box.
Modified ajax so uploading a file in the thumbnail image box overwrites the previous thumbnail url string.

Closes #19