Race condition creating assignments in non-Canvas LMSes

This exception might be reproducible in other ways but it's trivial to, in any non canvas LMS to:

- Create an assignment in the LMS, launch our tool, as this is a new assignment it will show the filepicker

- In another window launch the new assignment again. As this is still un-configured it will show the filepicker again

- Pick the content in either window.
- A new assignment will be created.
- Pick content in the the other window, you'll get this exception.

Not sure what the right solution is:

- Ignore this. It doesn't happen that often.
- The second configuration overrides the first one. Could potentially be confusing
- The second configuration gets ignored. Also, could be confusing.
- We show an error, "Hey you already configured this"

This last option could be extended with a prompt to decide what to do, which might complicate things.

---

Sentry Issue: [LMS-BACKEND-WX](https://hypothesis.sentry.io/issues/4424749778/?referrer=github_integration)

```
UniqueViolation: duplicate key value violates unique constraint "uq__assignment__resource_link_id"
DETAIL: Key (resource_link_id, tool_consumer_instance_guid)=(17688, moodle.uleth.ca) already exists.

File "sqlalchemy/engine/base.py", line 1965, in _exec_single_context
self.dialect.do_execute(
File "sqlalchemy/engine/default.py", line 921, in do_execute
cursor.execute(statement, parameters)
File "newrelic/hooks/database_psycopg2.py", line 61, in execute
return super(CursorWrapper, self).execute(sql, parameters, *args, **kwargs)
File "newrelic/hooks/database_dbapi2.py", line 37, in execute
return self.__wrapped__.execute(sql, parameters,

IntegrityError: (raised as a result of Query-invoked autoflush; consider using a session.no_autoflush block if this flush is occurring prematurely)
(psycopg2.errors.UniqueViolation) duplicate key value violates unique constraint "uq__assignment__resource_link_id"
DETAIL: Key (resource_link_id, tool_consumer_instance_guid)=(17688, moodle.uleth.ca) already exists.

[SQL: INSERT INTO assignment (resource_link_id, tool_consumer_instance_guid, copied_from_id, document_url, extra, is_gradable, title, description) VALUES (%(resource_link_id)s, %(tool_consumer_instance_guid)s, %(copied_from_id)s, %(document_url)s, %(extra)s, %(is_gradable)s, %(title)s, %(description)s) RETURNING assignment.id, assignment.created, assignment.updated]
[parameters: {'resource_link_id': '17688', 'tool_consumer_instance_guid': 'moodle.uleth.ca', 'copied_from_id': None, 'document_url': 'https://drive.google.com/uc?id=1JQ1E3WnoAYKRYEBf38ftTc5ovnzYKTZy&export=download', 'extra': '{"group_set_id": ""}', 'is_gradable': False, 'title': 'Vinyeta annotation'...
(39 additional frame(s) were not displayed)
...
File "lms/validation/__init__.py", line 92, in wrapper_view
return view(context, request)
File "lms/views/lti/basic_launch.py", line 117, in configure_assignment_callback
return self._show_document(assignment)
File "lms/views/lti/basic_launch.py", line 149, in _show_document
self.request.find_service(name="lti_h").sync(
File "lms/services/lti_h.py", line 48, in sync
self._group_info_service.upsert_group_info(
File "lms/services/group_info.py", line 30, in upsert_group_info
self._db.query(GroupInfo)
```