Enable staff debug everywhere except on detached blocks.

Staff markup was enabled on all block types in
https://github.com/edx/edx-platform/pull/10903

This works well inside the courseware, but it breaks layout of the course about
page, which is also an XModule, see:
https://github.com/edx/edx-platform/pull/10903#issuecomment-164266342

This commit disables staff markup/staff debug on all blocks except
blocks tagged with 'detached'. Detached blocks include course about
and info pages, static tabs.

lms/djangoapps/courseware/tests/test_module_render.py

@@ -1349,6 +1349,19 @@ class XmlViewInStudioTest(ViewInStudioTest):
         self.assertNotIn('View Unit in Studio', result_fragment.content)
 
 
+@XBlock.tag("detached")
+class DetachedXBlock(XBlock):
+    """
+    XBlock marked with the 'detached' flag.
+    """
+    def student_view(self, context=None):  # pylint: disable=unused-argument
+        """
+        A simple view that returns just enough to test.
+        """
+        frag = Fragment(u"Hello there!")
+        return frag
+
+
 @attr('shard_1')
 @patch.dict('django.conf.settings.FEATURES', {'DISPLAY_DEBUG_INFO_TO_STAFF': True, 'DISPLAY_HISTOGRAMS_TO_STAFF': True})
 @patch('courseware.module_render.has_access', Mock(return_value=True, autospec=True))
@@ -1404,6 +1417,28 @@ class TestStaffDebugInfo(ModuleStoreTestCase):
         result_fragment = module.render(STUDENT_VIEW)
         self.assertIn('Staff Debug', result_fragment.content)
 
+    @XBlock.register_temp_plugin(DetachedXBlock, identifier='detached-block')
+    def test_staff_debug_info_disabled_for_detached_blocks(self):
+        """Staff markup should not be present on detached blocks."""
+
+        descriptor = ItemFactory.create(
+            category='detached-block',
+            display_name='Detached Block'
+        )
+        field_data_cache = FieldDataCache.cache_for_descriptor_descendents(
+            self.course.id,
+            self.user,
+            descriptor
+        )
+        module = render.get_module(
+            self.user,
+            self.request,
+            descriptor.location,
+            field_data_cache,
+        )
+        result_fragment = module.render(STUDENT_VIEW)
+        self.assertNotIn('Staff Debug', result_fragment.content)
+
     @patch.dict('django.conf.settings.FEATURES', {'DISPLAY_HISTOGRAMS_TO_STAFF': False})
     def test_histogram_disabled(self):
         module = render.get_module(

lms/templates/staff_problem_info.html

@@ -6,7 +6,7 @@ from django.template.defaultfilters import escapejs
 
 ## The JS for this is defined in xqa_interface.html
 ${block_content}
-%if location.category in ['problem','video','html','combinedopenended','library_content']:
+%if 'detached' not in tags:
 %  if edit_link:
   <div>
       <a href="${edit_link}">Edit</a>

openedx/core/lib/xblock_utils.py

@@ -293,6 +293,7 @@ def add_staff_markup(user, has_instructor_access, disable_staff_debug_info, bloc
     staff_context = {
         'fields': field_contents,
         'xml_attributes': getattr(block, 'xml_attributes', {}),
+        'tags': block._class_tags,  # pylint: disable=protected-access
         'location': block.location,
         'xqa_key': block.xqa_key,
         'source_file': source_file,