InstructorFeedbackResultsPage: '14 Feb 1970, 00:00' was changed to 'unspecified (to be published manually)' #2626

Fixes #2626
I am a little bit confused about what you mean. 
As far as I understand, before a session is published when the option "Responses visible from" is set to "publish manually", this condition is satisfied:
```
 else if (bundle.feedbackSession.resultsVisibleFromTime.equals(Const.TIME_REPRESENTS_LATER)) {
        return "I want to manually publish the results.";
} 
```
Then, when the session is published, "Responses visible from" field contains '14 Feb 1970, 00:00'. 
So...I understood that in both cases - before publishing a session and after it was published - "Responses visible from"  should contain something like "Unspecified (to be published manually)".  
Is it right? Or, maybe, "Responses visible from" field should contain the actual time when the button "publish" was pressed?