Persisting Cell Language Mode

[mjsmagalhaes]

In the UI we can change the cell language to have editor and intellisense work better with what we are doing. An example is Sql Magic %%sql where we can change the cell language to sql to have better support.

But if you save and open the notebook again all code cells are set to python and we have to manually set them to sql.

The notebook should remember the language cell mode.

[rchiodo]

This should be a dupe of #4588