Add explicit links to git history for policies

Add explicit links to git history for policies

We don't do this for all pages, but specifically for policies that
already included the "Last update", it's friendly to have a link to the
full set of changes.

We still lave the "Last updated" field as manually updated, because we
don't want to update that one if we just fix a typo or similar, it
should be reserved for when we make acstual content updates to a policy.

This creates and uses a specific template tag to automate the creation
of the links (that can of course be used elsewhere as well if needed).