Featured Media for Visualizations

This is related to #1712 and #1775 

As discussed I have added the ability for users to feature their own images for visualizations. This is broken down as follows

1) the USER_ID of the media object generated by ImageMagick was not set, so it was never added to the list of media objects. (now fixed)

2) When there are no media objects for a visualization, everything would break. To solve this I have made a default for the case statement in which it decides which of the vis_XX_light.png images to use instead. 

Testers --
You should be able to create a visualization then delete the image.png it creates. The visualizations index page should not break after doing this. You should also be able to upload your own image here and feature it.