Add unique_together contraints for galaxy_id and invocation

Description

If the create_output_files method of an invocation is called multiple times. The output files already saved are not updated.
Instead new entries are created leading to multiple entries with the same galaxy_id.
Then, if update_output_files method is called, it will not work correctly because it assume that galaxy_id is unique.

To fix that:

• Add unique_together constraint for galaxy_id and invocation instance for GalaxyOutputFile model
• Modify create_output_files to take into account already saved output files of an invocation

Definition of Done

• New code added is tested
• Merge request has been revised and validated
• Wiki or documentation has been updated

Related issue(s)

None