Edx-toggles

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More improvements to monitoring of legacy waffle class imports.

* Add ``deprecated_incompatible_legacy_waffle_class`` custom attribute to any class (including subclasses), using the backward-incompatible imports that will be removed in 2.0.0.
* Add ``deprecated_compatible_legacy_waffle_class`` custom attribute to any class (including subclasses) using the legacy classes compatible with 2.0.0 imports, but which should be removed in 3.0.0 (or some future major version).
* Remove ``deprecated_edx_toggles_waffle`` custom attribute. In two cases, it was replaced by the new ``*_legacy_waffle_class`` custom attributes. In one case, it was replaced with the already existing and more appropriate ``deprecated_waffle_legacy_method`` custom attribute.

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* Improve monitoring of legacy Waffle class imports. We should watch for "edx_toggles.toggles.internal.waffle.legacy.WaffleSwitch" custom attributes.

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* Start the deprecation process of the waffle namespace classes:

* Introduce LegacyWaffleFlag, LegacyWaffleSwitch for use with namespaces.
* Begin deprecation/refactoring of namespacing code, including deprecation monitoring and warnings.
* Note: WaffleFlag and WaffleSwitch still use namespaces as well (for now).
* Introduce the ``toggles.__future__`` module for applications that need to be forward-compatible right away.

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* Fix cache-checking in WaffleSwitchNamespace

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* Backport ``override_waffle_switch`` test utility function from edx-platform

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* Fix missing ``module_name`` argument in ``SettingDictToggle`` constructor.
* Extract waffle classes from edx-platform and move them here following ADR `2 <docs/decisions/0002-application-toggle-state.rst>`__, `3 <docs/decisions/0003-django-setting-toggles.rst>`__, `4 <docs/decisions/0004-toggle-api.rst>`__. This does not introduce backward-incompatible changes, *yet*.
* Fix pinned requirements and incorrect root url that overrode edx-platform's.