Add Accept header negotiation to relevant API endpoints (#337)

* 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

internal/api/client/auth/token.go

@@ -19,10 +19,12 @@
 package auth
 
 import (
-	"github.com/sirupsen/logrus"
 	"net/http"
 	"net/url"
 
+	"github.com/sirupsen/logrus"
+	"github.com/superseriousbusiness/gotosocial/internal/api"
+
 	"github.com/gin-gonic/gin"
 )
 
@@ -41,6 +43,11 @@ func (m *Module) TokenPOSTHandler(c *gin.Context) {
 	l := logrus.WithField("func", "TokenPOSTHandler")
 	l.Trace("entered TokenPOSTHandler")
 
+	if _, err := api.NegotiateAccept(c, api.JSONAcceptHeaders...); err != nil {
+		c.JSON(http.StatusNotAcceptable, gin.H{"error": err.Error()})
+		return
+	}
+
 	form := &tokenBody{}
 	if err := c.ShouldBind(form); err == nil {
 		c.Request.Form = url.Values{}