Enhance "Managing security" contents

[StackScribe]

• Make this file include only the intro to the "Configure Global Security" screen and a discussion of the "Enable security" checkbox and perhaps "Disable remember me"
• Replace screenshot with current screenshot and include all sections -- perhaps breaking it up into the sections
• This file should follow the "Controller isolation" file.
• Move details about filling in the various sections to separate files that will be called in the order in which they appear on the screen: Security Realm, Authorization, Other Global Security settings.

@StackScribe is working on this.