Keyboard shortcut for submitting comments

Fixes #1546 

* Use ctrl+enter or cmd+enter to submit comments
* Added `isCommandKey(keyEvent)` method to RESUtils for developing other command key shortcuts

`commandDown` flag is used because otherwise we would need to rely on `e.metaKey`, which may or may not be the command key in all cases. 

Works in comments and the Wiki editor. Could probably be extended to posts if that is desired.