Reorganize security/permissions.md

Preview: http://spades.undo.it:8008/security/permissions/

Broadly speaking:

• Add screenshot to explain "Permission". 1 picture == 1000 words.
• Most of the topics/content raised before were good -- but missing some topics and wanting some clear org.
• Some of the headings confused me. Ex: "Extending and Implementing Permission Structure: Implementing Permissions in extensions" was really about setting permissions for routes and nav-items which happen to be defined in extensions.
• Add more content,eg
  • Naming conventions
  • Cross-over user-cases
  • More about APIv4