Centering equation in markdown and code cells.
Added new structure (input prompt and inner cell) to markdown cells.
Restructuration of null.tpl and addition of inner_cell classes.
Rename of input groups and use of self.block for e_in_prompt.
Deleting html clases for raw and unknown type cells.
Simplification of the logic.
Coming back raw cell to null template.
null.tplx regenerated.
Make the name of empty_in_prompt more explicit.
Fixed mathjax macro to be synced with the mathjax config in the live notebook.
Make the null templates compatible with the last Thomas's commit.
Merge pull request #4708 from damianavila/fix_indent_and_center

Fix indent and center