Re-add separator image

In dec223a56446e783f3d27b4e9e57127dc2cc2937 I removed this image
from Spotlight.

However, in alphagov/govuk_template#79 I've removed this image from the
GOV.UK template. The template is not the correct place for this image,
so add it to Spotlight.
Merge pull request #465 from alphagov/add-separator-image

Re-add separator image