Added more hotkeys, updated some descriptions

Added most of the hotkeys in [composer_controller.js](http://www.codecademy.com/assets/dist/controllers/composer_controller.js) (~line 20,000 of the [jsbeautified version](https://gist.github.com/JoahG/8246112)). Some of the previous descriptions were found to be inaccurate compared to the name of the hotkey combo. 

Some key combos were excluded as they aren't presently working
`ctrl + ,` for menu, the macro recording keys `ctrl + alt+ e` and `ctrl +shift + e`, a few others I can't remember.
Update can-i-navigate-codecademy-courses-with-keyboard-shortcuts.md
Update can-i-navigate-codecademy-courses-with-keyboard-shortcuts.md

`<em>`ed the mac support lines
fixing stuff...
Merge pull request #2 from JoahG/patch-10

fixing stuff...
Update can-i-navigate-codecademy-courses-with-keyboard-shortcuts.md
added separate key-blocks
Update can-i-navigate-codecademy-courses-with-keyboard-shortcuts.md
modern macs still support insert
upcased keys, added arrows
removed duplicates
light cleanup
silly typo
clean up formatting and add missing closing tag
Merge branch 'patch-2' of github.com:gerbal/help.codecademy.com into keyboard-shortcuts

Conflicts:
	published/general/can-i-navigate-codecademy-courses-with-keyboard-shortcuts.md
update layouts and separate Codecademy specific and universal shortcuts into 2 tables
remove Chrome specific shortcut
add punctuation
update universal table text formatting
add mac+pc articles for shortcut article and remove combined article