Enable instructors to grab a learner's session

show_in_read_only_mode is an attribute that is False by default, and
means that when an instructor is masquerading (that is, using the
LMS with "View this course as: Specific learner" option), the
XBlock is replaced by the text "This type of component cannot be shown
while viewing the course as a specific student"
(in lms/djangoapps/courseware/masquerade.py).

In our case, however, we do want an instructor to be able to
conveniently take over another learner's session.

Thus, set show_in_read_only_mode to True, so that the XBlock is
rendered even when masquerading.

hastexo/hastexo.py

@@ -107,6 +107,7 @@ class HastexoXBlock(XBlock,
     has_children = True
     icon_class = 'problem'
     block_settings_key = SETTINGS_KEY
+    show_in_read_only_mode = True
 
     @classmethod
     def parse_xml(cls, node, runtime, keys, id_generator):