* Add local user and post count to nodeinfo responses
This fixes#1307 (at least partially). The nodeinfo endpoint should now
return the total users on an instance, along with their post count.
* Update NodeInfoUsers docstring and swagger yaml file
* start centralizing negotiation logic for API
* swagger document nodeinfo endpoint
* go fmt
* document negotiate function
* use content negotiation
* tidy up negotiation logic
* negotiate content throughout client api
* swagger
* remove attachment on Content
* add accept header to test requests
