Admin can unlock copy

Admin will be able to 'unlock' copy that need corrections, in order to allow student to access to his copy unless he has got the required number of correction.

- to Acess to the unlock button, the dropzone must be in the closed state
- the copy need at least 1 correction to be unlocked.

An unlocked copy cannot be corrected anymore by other people  excepts dropzone administrator's.


![unlock button](https://cloud.githubusercontent.com/assets/3479942/3144675/01dd4890-ea28-11e3-855c-1071e17e61ec.png)

![unlock_1](https://cloud.githubusercontent.com/assets/3479942/3144679/07f8bdae-ea28-11e3-96f4-46186ab9b157.png)