Make cmd+enter save note

[therohitdas]

In the Mac app, when writing a note, it will be better to use cmd+enter to save the note. Right now, we need to click the save button.

cmd+enter on Mac and ctrl+enter everywhere else will increase the user's speed because the hands are already on the keyboard when typing the note. Also, it is the default way to complete interaction with textboxes in many places including GitHub, I will press cmd+enter to post this issue.