[db] Enforce order in which migrations are applied

In 9e33378 we should have had a migration which does
AlterField on LinkReference.test_case_run to update the FK
constraint to the new `TestExecution` name!

Didn't catch this b/c Django was ordering migrations in such a
way that there wasn't a problem. Also the DB engines probably
automatically handled the model name change for all foreign key
fields.

However with the next commit the order in which migrations are
applied changes (b/c we add new model referencing TestExecution)
and we started seeing failures b/c linkreference.0001 started
being applied after testruns.0006 at which point there is no
`TestCaseRun` model to reference as the foreign key in the first
place.

FYI @asankov

tcms/testruns/migrations/0006_rename_test_case_run_to_test_execution.py

@@ -24,6 +24,9 @@ class Migration(migrations.Migration):
         ('testcases', '0008_notifications_default_true'),
         ('management', '0005_order_by_name'),
         ('testruns', '0005_remove_unused_fields'),
+        # enforces migrations order b/c linkreference.0001
+        # refers to TestCaseRun which we rename here
+        ('linkreference', '0001_squashed'),
     ]
 
     operations = [