Move docimpact from created hook to merged

* .../files/gerrit/change-merged: Call the docimpact script from
this hook.

* .../templates/gerrit_patchset-created.erb: Stop calling the
docimpact script from this hook.

Change-Id: I9a2d6e33ec35eb94ae14f7d2b17ead52bb9a2019
Merge "Move docimpact from created hook to merged"