Fix download site layout problem

Fix download site layout problem when Pre-Release column is empty.

* Add CSS for an empty list item
* Print empty list if there are no Pre-Release files
* Pad Pre-Release file list if there are fewer Pre-Release files
  than Release files

Change-Id: If76cf9e74a8f5068f0f0f785279b87460b962962
Closes-Bug: 1358973
Merge "Fix download site layout problem"