Feincms3

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- Dropped compatibility with Python 2.
- Fixed ``AbstractPage.save()`` to actually detect page moves correctly
again. Calling ``save()`` in a transaction was a bad idea because it
messed with MPTT's bookkeeping information. Depending on the
transaction isolation level going back to a clean slate *after*
``clean()`` proved much harder than expected.

~~~~~~~~~~~~~~~~~~~

- Added ``feincms3.apps.reverse_fallback`` to streamline reversing with
fallback values in case of crashes.
- The default template renderer context
(``TemplatePluginRenderer.register_template_renderer``) contains now the
plugin instance as ``plugin`` instead of nothing.
- Make django-mptt-nomagic_ a required dependency, by depending on the fact
that nomagic always calls ``Page.save()`` (django-mptt_ does not do that
when nodes are moved using ``TreeManager.node_move``, which is used in the
draggable mptt admin interface. Use a ``node_moved`` signal listener which
calls ``save()`` if the ``node_moved`` call includes a ``position`` keyword
argument if you can't switch to django-mptt-nomagic_ for some reason.

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- Removed all dependencies from ``install_requires`` to make it easier
to replace individual items.
- Enabled the use of ``i18n_patterns`` in ``ROOT_URLCONF`` by importing
and adding the urlpatterns contained instead of ``include()``-ing the
module in ``apps_urlconf``.
- Modified the cleansing configuration to allow empty ``<a>`` tags
(mostly useful for internal anchors).
- Fixed crash when adding a page with a path that exists already (when
not using a statich path).

~~~~~~~~~~~~~~~~~~~

- Updated the translation files.
- Fixed crashes when path of pages would not be unique when moving
subtrees.

~~~~~~~~~~~~~~~~~~~

- Fixed a crash where apps without ``required_fields`` could not be
saved.
- Added a template snippet based renderer for plugins.
- Prevented adding the exact same application (that is, the same
``app_instance_namespace``) more than once.

~~~~~~~~~~~~~~~~~~~

- Made application instances (``feincms3.apps``) more flexible by
allowing programmatically generated instance namespace specifiers.