Allow direct editing of build policy output

[dgabrahams]

It will allow greater versatility and customisation if the build_policy function was made to return the csp variable before it is changed into policy_parts (https://github.com/mozilla/django-csp/blob/main/csp/utils.py#L90). It would be required to refactor the addition of nonce and final string construction (https://github.com/mozilla/django-csp/blob/main/csp/utils.py#L111) to new functions. Developers could then hook into the output of build_policy ordering/appending as required and then returning the nonce and string construction function outputs as required.