Add endpoints for source and translation string details.

Required some changes to how endpoints are specified so that earlier endpoints would take precedence, and so that different query string parameters can be specified.
> some applications restrict the number of elements that can be read (written) in a single operation, maybe we should consider the same.

I agree completely. We can figure out a sensible limit once we see what the editor tends to use normally. I suspect it will also force better async design in the editor, by making us think about breaking up requests that would be too large.