[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/user/star.go

@@ -92,6 +92,8 @@ func GetMyStarredRepos(ctx *context.APIContext) {
 	if err != nil {
 		ctx.Error(http.StatusInternalServerError, "getStarredRepos", err)
 	}
+
+	ctx.SetTotalCountHeader(int64(ctx.User.NumStars))
 	ctx.JSON(http.StatusOK, &repos)
 }