Fixed #14516 -- Extract methods from removetags and slugify template filters

Patch by @jphalip updated to apply, documentation and release notes
added.

I've documented strip_tags as well as remove_tags as the difference
between the two wouldn't be immediately obvious.

tests/regressiontests/utils/html.py

@@ -146,3 +146,12 @@ class TestUtilsHtml(unittest.TestCase):
         )
         for value, output in items:
             self.check_output(f, value, output)
+
+    def test_remove_tags(self):
+        f = html.remove_tags
+        items = (
+            ("<b><i>Yes</i></b>", "b i", "Yes"),
+            ("<a>x</a> <p><b>y</b></p>", "a b", "x <p>y</p>"),
+        )
+        for value, tags, output in items:
+            self.assertEquals(f(value, tags), output)