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.
Allow '.', '+.', '+', '-.', and '-' even though without numbers, they cannot be parsed.
remove unneeded `width: 100%` firefox workaround

this is no longer needed with `display: flex`
add modal_stretch class

for stretching a dialog to fill the screen
use modal_stretch on keyboard shortcut dialog
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
Add coding magic comment to nbconvert Python template

Closes gh-5470
Merge pull request #5471 from takluyver/i5470

Add coding magic comment to nbconvert Python template
Minor corrections of git-hooks setup instructions
Merge pull request #5473 from YS-L/githooks

Minor corrections of git-hooks setup instructions
Merge pull request #5449 from minrk/quickhelp-stretch

Stretch keyboard shortcut dialog
Merge pull request #5444 from jdfreder/numparse

Fix numeric verification for Int and Float text widgets.
Merge pull request #5436 from ivanov/cm-shorcuts-quickhelp

CodeMirror shortcuts in QuickHelp
Merge branch 'master' of github.com:ipython/ipython