#3694 - get preview working & tested for offline-contribution receipt

...ribute/WorkflowMessage/Contribution/BasicContribution.php

CRM/Contribute/WorkflowMessage/ContributionTrait.php

CRM/Financial/BAO/Order.php

Civi/Test/ExampleData/Contribution/Euro5990.php

sql/civicrm_generated.mysql

tests/phpunit/CRM/Contribute/Form/ContributionTest.php

tests/phpunit/api/v4/Entity/WorkflowMessageTest.php

...s/message_templates/contribution_offline_receipt_html.tpl

...s/message_templates/contribution_offline_receipt_text.tpl