strip_all_tags_but_br_filter would return HTML safe string.

Now returning HTML safe string to template instead
of just bleaching string for br tags.

LEARNER-3930

openedx/core/djangolib/markup.py

@@ -37,7 +37,7 @@ def HTML(html):                                 # pylint: disable=invalid-name
 
 def strip_all_tags_but_br(string_to_strip):
     """
-    Strips all tags from a string except <br/>
+    Strips all tags from a string except <br/> and marks as HTML.
 
     Usage:
         <%page expression_filter="h"/>
@@ -53,4 +53,4 @@ def strip_all_tags_but_br(string_to_strip):
     string_to_strip = decode.utf8(string_to_strip)
     string_to_strip = bleach.clean(string_to_strip, tags=['br'], strip=True)
 
-    return string_to_strip
+    return HTML(string_to_strip)

openedx/core/djangolib/tests/test_markup.py

@@ -90,3 +90,12 @@ class FormatHtmlTest(unittest.TestCase):
 
         self.assertIn('<br>', rendered_template)
         self.assertNotIn('<script>', rendered_template)
+
+    def test_strip_all_tags_but_br_returns_html(self):
+        """
+        Verify filter returns HTML Markup safe string object
+        """
+
+        html = strip_all_tags_but_br('{name}<br><script>')
+        html = html.format(name='Rock & Roll')
+        self.assertEqual(html.decode(), u'Rock &amp; Roll<br>')