Investigate possibilities of marking an issue referenced as a task in an epic as done

[MatousJobanek]

An epic issue can contain multiple tasks either in a description or in any of the comments. An example is here: #68
Every task has an issue reference in its description. When an issue is closed it would be good if the task could be automatically marked as done.
Investigate if it is possible to find the references to the closed issue and how hard would be marking the tasks as done via the API