[#903] Remove questionId uniqueness check at save

* We were validating the uniqueness of the questionId when the user clicks 'save'. This was waiting a second. If during that second the users opens another question for edit, the properties were copied.
[#906] Release Notes v1.7.6.1
Merge pull request #907 from akvo/issue/906-release-notes

[#906] Release Notes v1.7.6.1
Merge branch 'release/1.7.6'