Split buildPageTargeting method out from dfp module

No reason for this to be bundled into the dfp module
Going by https://github.com/guardian/frontend/commit/9a13b04a21d6548eca0c375e588c68ec3703fd15#diff-fea3de1ebf1ba71ca7fe06d0fa46dd3eL556 it's for series fronts

@kelvin-chappell, can you remember