Fixed #23831 -- Supported strings escaped by third-party libs in Django.

Refs #7261 -- Made strings escaped by Django usable in third-party libs.

The changes in mark_safe and mark_for_escaping are straightforward. The
more tricky part is to handle correctly objects that implement __html__.

Historically escape() has escaped SafeData. Even if that doesn't seem a
good behavior, changing it would create security concerns. Therefore
support for __html__() was only added to conditional_escape() where this
concern doesn't exist.

Then using conditional_escape() instead of escape() in the Django
template engine makes it understand data escaped by other libraries.

Template filter |escape accounts for __html__() when it's available.
|force_escape forces the use of Django's HTML escaping implementation.

Here's why the change in render_value_in_context() is safe. Before Django
1.7 conditional_escape() was implemented as follows:

    if isinstance(text, SafeData):
        return text
    else:
        return escape(text)

render_value_in_context() never called escape() on SafeData. Therefore
replacing escape() with conditional_escape() doesn't change the
autoescaping logic as it was originally intended.

This change should be backported to Django 1.7 because it corrects a
feature added in Django 1.7.

Thanks mitsuhiko for the report.

django/utils/html.py

@@ -44,6 +44,10 @@ def escape(text):
     """
     Returns the given text with ampersands, quotes and angle brackets encoded
     for use in HTML.
+
+    This function always escapes its input, even if it's already escaped and
+    marked as such. This may result in double-escaping. If this is a concern,
+    use conditional_escape() instead.
     """
     return mark_safe(force_text(text).replace('&', '&amp;').replace('<', '&lt;')
         .replace('>', '&gt;').replace('"', '&quot;').replace("'", '&#39;'))
@@ -76,6 +80,9 @@ escapejs = allow_lazy(escapejs, six.text_type, SafeText)
 def conditional_escape(text):
     """
     Similar to escape(), except that it doesn't operate on pre-escaped strings.
+
+    This function relies on the __html__ convention used both by Django's
+    SafeData class and by third-party libraries like markupsafe.
     """
     if hasattr(text, '__html__'):
         return text.__html__()