Keyboard shortcut for "Comment" action

[rachel-fenichel]

In #224 @kmcnaught said ""Comment": probably needs a shortcut key, once comments are keyboard-navigable. I imagine many people don't comment, but once you start commenting, you probably do it quite a lot."

Steps to resolving this issue:

• Decide on an appropriate key combination (in discussion on this issue)
• Turn the comment context menu item into an action with both context menu and keyboard triggers, following the pattern established in actions/delete.ts and friends