provide a way to disable a cell

[trebor]

Is your feature request related to a problem? Please describe.

often while debugging i need to remove cells from evaluation.  currently to do this i need to:

1. if it is a markdown cell, convert it to a js cell
2. open the cell
3. select the content and comment it out

and to restore:

4. if it was formally markdown cell, convert it from js back to markdown
5. open the cell
6. select the content and comment it in

throughout this process, i risk spurious edits.

Describe the solution you'd like

provide a way to disable/enable cells. disabling a cell would grey it out, lock editing, and remove it from evaluation.

Additional context

i might note that the ⌥-D shortcut is not occupied. :)

[CobusT]

Related: #381

[trebor]

related: #495

[CobusT]

Marking as a duplicate.