tickets/ticket comment "external_ref and external_system" fields can be added more than once

[jasonpagetas]

When adding a ticket comment from the API a ticket that has the same external reference and external system should fail as the object already exists, this is not happening and the same ticket comment can be added multiple times.

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Details

both ticket and ticket comments have fields external_ref and external_system. these fields are for use when importing tickets and comments from an external system. these must be validated as unique.

Requirements

• [x] validate ticket as combined external_ref and external_system as unique per row
• [x] validate ticket comment as combined external_ref and external_system as unique per row

[jon-nfc]

this'll be added to milestone v1.2.