Completed Epics Should Not be accepted in the Epic Link Field

Currently, when you close an Epic/mark it as Epic Status set to 'Done', it will still be allowed when typed as the Epic Link field of a story.

Steps to Reproduce

1. close an epic and mark Epic Status to Done. Set it to done in a postfunction
2. edit a new feature issue, input issue key of the closed epic on epic link field
3. system will show an error:
Epic with key ... does not exist.
4. ignore this error and click update, then epic link is save.

Expected Behaviour

Epic link of closed Epic should not be accepted. Should not allow save

Actual Behaviour

Results in issue with Epic Link of closed Epic