Bad values for launched/concluded when times are unknown

[PeterJCLaw]

For experiments where the launched or concluded times are unknown, the web API exposes the string 'None' rather than a JSON null value. This makes parsing the data harder than necessary.

It seems that this is a result of the naive JSON preparation of the data done in

jacquard/jacquard/service/endpoints.py

Lines 97 to 98 in d29abba

	"launched": str(experiment_config.launched),
	"concluded": str(experiment_config.concluded),

I think the fix is to either have these check for None before calling str or to push the datetime string conversion into the JSON encoder so that types are handled generically.