add observer logos
display observer logo on observer front
point observer logo to observer front
point to the right place
Merge branch 'observer-logo' of github.com:guardian/frontend into observer-logo

Conflicts:
	common/app/views/fragments/header.scala.html
make observer logos the same colour as the nav
don't nest single pseudo el rule
Merge pull request #5159 from guardian/observer-logo

Add the Observer logo the Observer front