bugfix/migration0073

[gecBurton]

Context

migration 0073 is not running in production

this is due to a subtly in the way django creates many-to-many tables.

previously the many to many relationship was established via the Citation model that had a foreign key to both the chat_message and file, where there maybe many citations with referencing the same chat_message and file. The django ManyToMany relationship creates a very similar through model but it enforces a uniqueness constraint on the foreign keys which caused the error.

Changes proposed in this pull request

• 306f9f0: extend test by adding two citations with the same chat_message and file, note that this fails with the same error we see in prod could not create unique index "redbox_core_chatmessage__chatmessage_id_file_id_bd4360c8_uniq"
• bafde4b fix the migration by removing the "duplicate" citations

Guidance to review

Relevant links

Things to check

• I have added any new ENV vars in all deployed environments
• I have tested any code added or changed
• I have run integration tests