[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/repo/issue_watch.go

@@ -38,7 +38,7 @@ func IssueWatch(ctx *context.Context) {
 				log.Trace("Permission Denied: Not logged in")
 			}
 		}
-		ctx.Error(403)
+		ctx.Error(http.StatusForbidden)
 		return
 	}