cell metadata evaluable == false

Originally motivated by reading up on nbgrader and then some work for CoCalc, I've came across the metadata fields `"deletable": false` and `"editable":false`. (reference: [nbgrader test_lockcells.py](https://github.com/jupyter/nbgrader/blob/master/nbgrader/tests/preprocessors/test_lockcells.py#L15))

However, I was wondering if there are also plans for a third `"evaluable":false` metadata flag? If that's set, the cell can't be evaluated and the stored cell output stays as it is.

Start by reading nbgrader's test_lockcells.py reference and the issue's proposed deletable/editable metadata behavior. Determine where Jupyter Notebook handles cell metadata and evaluation, then define the behavior for evaluable:false, including preserving stored output; the issue is done when that behavior is implemented and covered by tests.

Written by the indexing model from the issue text.