refactor(if): Deprecate ifnot tag

CHANGELOG.md

@@ -13,6 +13,8 @@
   to be able to test. Now we have access to `@testable` these can correctly be
   private.
 
+- `{% ifnot %}` tag is now deprecated, please use `{% if not %}` instead.
+
 ### Enhancements
 
 - Variable lookup now supports introspection of Swift types. You can now lookup

docs/builtins.rst

@@ -164,6 +164,8 @@ Will be treated as:
 ``ifnot``
 ~~~~~~~~~
 
+.. note:: ``{% ifnot %}`` is deprecated. You should use ``{% if not %}``.
+
 .. code-block:: html+django
 
     {% ifnot variable %}