CodeMirror shortcuts in QuickHelp

Our users shouldn't need an internet connection to look at the code
mirror shortcuts. This PR places the CM keyboard shortcuts that are
currently documented on our website's rendered docs, and places them
inside the keyboard shortcuts quickhelp.
Hi, I'm a Mac. And I'm a PC.
completed both sets of platform-specific shortcuts
remove no-op placeholder edit mode "shortcuts"
lower-cased single letter keys in shortcuts
move hyphen to cmd_ctrl
remove cm_keyboard.rst and OS-level shortcuts
ok, removed last few unnecessary shortcuts
Merge pull request #5436 from ivanov/cm-shorcuts-quickhelp

CodeMirror shortcuts in QuickHelp