Remove exempt orgs thumbnails

• Remove unnecessary code from not_live page
• Remove nested css for that section on for exempt orgs
• Update styles for adjacent elements to fix layout
• Remove unsed code from helper
• Remove rake task for generating thumbnails

https://www.agileplannerapp.com/boards/105200/cards/5218
Removed unused screenshots
Merge pull request #1820 from alphagov/remove-external-org-thunmbnail

PO accepted according to @russelthorn