Make topic deletion logic transactional

[kevintab95]

Describe the bug

The code for topic deletion involves deletion of several associated models. See below:

oppia/core/domain/topic_services.py

Lines 988 to 999 in c57292a

	topic_rights_model = topic_models.TopicRightsModel.get(topic_id)
	topic_rights_model.delete(
	committer_id, feconf.COMMIT_MESSAGE_TOPIC_DELETED,
	force_deletion=force_deletion)
	# Delete the summary of the topic (regardless of whether
	# force_deletion is True or not).
	delete_topic_summary(topic_id)
	topic_model = topic_models.TopicModel.get(topic_id)
	for subtopic in topic_model.subtopics:
	subtopic_page_services.delete_subtopic_page(
	committer_id, topic_id, subtopic['id'])

This issue is to ensure that the deletion of topic and related models happen in a transactional manner i.e. if an operation fails, none of the associated models should be committed to the datastore. This prevents data inconsistencies / corruption on the server.

URL of the page where the issue is observed.

NA

Steps To Reproduce

NA -- no changes to functionality.

Expected Behavior

NA

Screenshots/Videos

No response

What device are you using?

Desktop

Operating System

Linux

What browsers are you seeing the problem on?

No response

Browser version

No response

Additional context

No response

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