Fixed #25135 -- Deprecated the contrib.admin allow_tags attribute.

Thanks Jaap Roes for the idea and initial patch.

docs/internals/deprecation.txt

@@ -265,6 +265,9 @@ details on these changes.
 * The warning that :class:`~django.core.signing.Signer` issues when given an
   invalid separator will become an exception.
 
+* Support for the ``allow_tags`` attribute on ``ModelAdmin`` methods will be
+  removed.
+
 .. _deprecation-removed-in-1.9:
 
 1.9