Currently it's possible to "Unset as Default" the default quality gate, leaving the instance with no default quality gate. This ability should be removed.
Moreover, it should not be possible to delete the default quality gate.
The following web services must fail :
- api/qualitygates/unset_default => this action should not exist anymore
- api/qualitygates/destroy => it should fail to delete default quality gate
The "Unset as Default" button must be removed