[API] generalize list header (#16551)

* Add info about list endpoints to CONTRIBUTING.md

* Let all list endpoints return X-Total-Count header 

* Add TODOs for GetCombinedCommitStatusByRef

* Fix models/issue_stopwatch.go

* Rrefactor models.ListDeployKeys

* Introduce helper func and use them for SetLinkHeader related func

routers/api/v1/repo/subscriber.go

@@ -52,5 +52,7 @@ func ListSubscribers(ctx *context.APIContext) {
 	for i, subscriber := range subscribers {
 		users[i] = convert.ToUser(subscriber, ctx.User)
 	}
+
+	ctx.SetTotalCountHeader(int64(ctx.Repo.Repository.NumWatches))
 	ctx.JSON(http.StatusOK, users)
 }