Update Guardian/Observer logo

A few things:
* Consistent size and positioning for Guardian and Observer logos in the header (both now 320px x 60px)
* Removed observer-logo-160-header.svg as no longer required (I think this is correct @sndrs)
* Use -320 suffix for larger logo SVG (might be controversial, but consistent with other image files)
* Smaller version of logo resized using width and height attributes (uses same path co-ordinates as larger version)
* Remove guardian-logo@2x as no longer used (/cc @phamann, @kaelig)
* Update no-image.png to use ‘batwing’
* Update logo used for RSS