Allow Admin to set conditions on completing sprint

At the moment, the following are the condition to complete sprint:

1. Have the right permission.
2. All sub-task belong to completed issues in that sprints are complete.

It would be great to have the ability to set our own condition on completing sprint. The options for the implementation could be one of the following:

• The complete sprint button should be disabled as long as the conditions have not been fulfilled yet.
• Or an error message should be popped up when completing the sprint showing the conditions need to be fulfilled.