Replace hand-made json validator by validateJSON utility

[thamara]

In #946 a new formal json validator was added. The task here is to replace the current way of validating the json file in js/import-export.js by the one implemented in js/validate-json.js.

[nehagupta1504]

@thamara can you point to me the line no. in js/import-export.js which you are asking to add validation or replacing the older way

[tupaschoal]

@nehagupta1504 to me it looks like the validEntry function should be replaced.