JSON output allowing numbers that begin with zero

[at88mph]

As per JSON.org, numbers may not begin with a zero, but when listing users, some IDs begin with a zero, but are written out as a number datatype.

To reproduce:

• Authenticated request to /ac/users/ with the Accept: application/json header set.
• First returned value has identities/$/@type=HTTP numeric value that begins with zero.

The JSON parser in the browser fails to load this, and so we cannot make a proper CORS request.

[at88mph]

This is an issue with the JsonOutputter in cadc-util, so am moving this to the core repository.