Display keyboard shortcut appropriate to OS

Closes #348

* Changed markdown.hbs so that all keyboard shortcuts are written in Windows format (e.g. Ctrl + B)
* Changed markdown.hbs to use Ctrl/Cmd + Shift + K for `code` shortcut. This is a change for Mac functionality from previous shortcut of Cmd + K
* Added afterRender callback to the Markdown modal that replaces instances of Ctrl with ⌘