Revert "Merge pull request #176 from alphagov/shared-mustache"

The Rake task isn't running, which blocks deployment.

This reverts commit 45c8c2918bb54499a5dd78959910545d533902bc, reversing
changes made to 3ecab419454388469b425c2d3353ab531d995e53.
Revert "Revert "Merge pull request #176 from alphagov/shared-mustache""

This reverts commit 2bcb029d6e99c2fcf1c40ae0d0597e235af92baf.

It also moves the shared_mustache outside of the asset group. It it not
just required for assets and was causing preview deploy to fail.