[refactor] replace int with httpStatusCodes (#15282)

* replace "200" (int) with "http.StatusOK" (const)

* ctx.Error & ctx.HTML

* ctx.JSON Part1

* ctx.JSON Part2

* ctx.JSON Part3

routers/swagger_json.go

@@ -5,6 +5,8 @@
 package routers
 
 import (
+	"net/http"
+
 	"code.gitea.io/gitea/modules/base"
 	"code.gitea.io/gitea/modules/context"
 	"code.gitea.io/gitea/modules/log"
@@ -19,6 +21,6 @@ func SwaggerV1Json(ctx *context.Context) {
 	ctx.Resp.Header().Set("Content-Type", "application/json")
 	if err := t.Execute(ctx.Resp, ctx.Data); err != nil {
 		log.Error("%v", err)
-		ctx.Error(500)
+		ctx.Error(http.StatusInternalServerError)
 	}
 }