Manheim-c7n-tools

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* Add ``m2r`` package as dependency, for Sphinx docs builds.

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* If the ``policy_source_paths`` configuration option is specified, have policygen include a column showing which source(s) a policy came from in ``policies.rst``.
* Fix bug in ``dryrun-diff`` step where it would fail on an initial, empty S3 bucket.

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* Previously, manheim-c7n-tools set c7n-mailer's ``templates_folders`` configuration option to ``/manheim_c7n_tools/manheim_c7n_tools/mailer-templates`` if that directory exists, or to the absolute path to a ``mailer-templates`` directory inside the ``manheim_c7n_tools`` installation otherwise. This behavior was largely based on the legacy hard-coded templates directory. Now that c7n-mailer template locations are more flexible, this behavior has been updated to (in order of evaluation):

* Use the ``templates_folders`` option from the ``mailer_config`` section of ``manheim-c7n-tools.yml``, if present. Otherwise, start with an empty list.
* Prepend ``./mailer-templates`` if it exists.
* Prepend ``/manheim_c7n_tools/manheim_c7n_tools/mailer-templates`` to the list, if it exists.
* Prepend ``mailer-templates`` directory inside the ``manheim_c7n_tools`` installation, if it exists.