Bugfix/null translation

Created by: mira13

#478 Description

Bugfix, that makes canonical_translation_id NULL on translation delete

Use case

When translation is removed, links to the translation still thers in transcript table

Benefits

Data consistency for translation table

Possible Drawbacks

Not found

Testing

Unit tests are added for this case

If so, do the tests pass/fail?

All tests pass, with travis as well