(into master) Bug 978338 - Update new tab site rendering for fixed ratio tile images
implement fixed size tiles and centered grid
Remove title background
Move title sizing to content css and fix the line height to prevent mouseover gaps.
Clean up newtab-cell changes and remove unnecessary margin-ends.
Remove unused rows css.
(into bug978338) Bug 980014 - Allow new tab grid layout to reduce rows/columns if the window can't fit 3x3
Allow up to the maximum rows and columns and have layout flow the tiles without explicit rows.