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. 
Wouldn't it be simpler just to call `RESUtils.isCommandKey(e)` here and remove the `commandDown` variable entirely?