Fix Bug 1019262, implement graphs for adu by signature

@peterbe and/or @rhelmer r?

I did not include a test for the new view function adu_by_signature_json. Not exactly sure what to test there, cause the form and the tests for the form will ensure the data is present and valid so, all that the middleware response can really be is data or no data.

If there is no data, the JS handles that case. I might very well be overlooking something but, I gotta open this PR sometime :)