Reconsider Sage "practice" cells

[rbeezer]

A Sage code element, with @type="practice" gets its own HTML class (sage-practice) which means it is not linked against other cells on the page. But in other formats (SageMathCloud worksheet, Jupyter notebook) all these cells will know their results.

Sage Cell class controls both linking and language, so we are partially handcuffed here.

[kcrisman]

See also discussion about Sage cell properties at https://groups.google.com/g/pretext-support/c/rdIi42AJ8cg