Allow SGA blocks to be used in Libraries v2 context [FC-0076] (#363)

* fix: use context_key instead of course_id

as it means the same thing (in the course context), and is provided as
part of xblock.core

* fix: work around errors preventing block preview in library context

* `start` attribute does not exist in library context
* catch errors for fields scoped outside "content" or "settings"

* chore: bump package version

edx_sga/__init__.py

@@ -2,4 +2,4 @@
 Module for StaffGradedAssignmentXBlock.
 """
 
-__version__ = "0.25.0"
+__version__ = "0.25.1"

edx_sga/sga.py

@@ -644,8 +644,10 @@ def block_id(self):
     def block_course_id(self):
         """
         Return the course_id of the block.
+
+        Note: if this block is used in a Content Library, the returned ID will be the library's ID.
         """
-        return str(self.course_id)
+        return str(self.context_key)
 
     def get_student_item_dict(self, student_id=None):
         """
@@ -698,13 +700,18 @@ def update_staff_debug_context(self, context):
         """
         Add context info for the Staff Debug interface.
         """
-        published = self.start
+        published = getattr(self, 'start', None)
         context["is_released"] = published and published < utcnow()
         context["location"] = self.location
         context["category"] = type(self).__name__
-        context["fields"] = [
-            (name, field.read_from(self)) for name, field in self.fields.items()
-        ]
+        context["fields"] = []
+
+        for name, field in self.fields.items():
+            try:
+                context["fields"].append((name, field.read_from(self)))
+            # Library blocks only support the content and settings scopes
+            except NotImplementedError:
+                pass
 
     def get_student_module(self, module_id):
         """