Fixed #21188 -- Introduced subclasses for to-be-removed-in-django-XX warnings

Thanks Anssi Kääriäinen for the idea and Simon Charette for the
review.

django/template/defaulttags.py

@@ -17,6 +17,7 @@ from django.template.base import (Node, NodeList, Template, Context, Library,
     render_value_in_context)
 from django.template.smartif import IfParser, Literal
 from django.template.defaultfilters import date
+from django.utils.deprecation import RemovedInDjango18Warning
 from django.utils.encoding import force_text, smart_text
 from django.utils.safestring import mark_safe
 from django.utils.html import format_html
@@ -591,7 +592,7 @@ def cycle(parser, token, escape=False):
             "'The `cycle` template tag is changing to escape its arguments; "
             "the non-autoescaping version is deprecated. Load it "
             "from the `future` tag library to start using the new behavior.",
-            DeprecationWarning, stacklevel=2)
+            RemovedInDjango18Warning, stacklevel=2)
 
     # Note: This returns the exact same node on each {% cycle name %} call;
     # that is, the node object returned from {% cycle a b c as name %} and the
@@ -739,7 +740,7 @@ def firstof(parser, token, escape=False):
             "'The `firstof` template tag is changing to escape its arguments; "
             "the non-autoescaping version is deprecated. Load it "
             "from the `future` tag library to start using the new behavior.",
-            DeprecationWarning, stacklevel=2)
+            RemovedInDjango18Warning, stacklevel=2)
 
     bits = token.split_contents()[1:]
     if len(bits) < 1: