Drop unused files to slim down grappelli_safe

[dsanders11]

I identified several files that I believe are not necessary to grappelli_safe anymore.

• templates/admin/template_validator.html
  • This is a remanent from Django 1.2 era and was dropped via ticket 12872
• templates/admin/includes_grappelli/messages.html
• templates/admin/includes_grappelli/help.html
• templates/admin/includes_grappelli/navigation.html
  • These aren't referenced anywhere in grappelli_safe and I don't see where they would be used
• templates/admin/404.html
• templates/admin/500.html
• templates/admin/invalid_setup.html
• templates/registration/logged_out.html
  • These are all close enough to mainline Django there's no need to duplicate them and not get updates to them for free
• static/grappelli/css/documentation.css
  • Appears to be an unused CSS file, can't find it used anywhere and the classes it references aren't used

A quick double check of the above is not a bad idea, but I haven't observed any changes after dropping these files.

[stephenmcd]

That's awesome, thanks David.