Screenshot previews displayed on dashboard

Updated projects on the dashboard to display preview screenshots when they
have been previewed at least once. There are 3 images of different sizes,
to view each image mouse over the large image. Before a project is previewed it
will display the default origami image.
Updated variable names
Adding route to screenshots
Removed unnecessary variables
Merge pull request #492 from adaptlearning/preview-on-dashboard

Preview on dashboard