[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/milestone.go

@@ -57,7 +57,7 @@ func ListMilestones(ctx *context.APIContext) {
 	//   "200":
 	//     "$ref": "#/responses/MilestoneList"
 
-	milestones, err := models.GetMilestones(models.GetMilestonesOption{
+	milestones, total, err := models.GetMilestones(models.GetMilestonesOption{
 		ListOptions: utils.GetListOptions(ctx),
 		RepoID:      ctx.Repo.Repository.ID,
 		State:       api.StateType(ctx.FormString("state")),
@@ -72,6 +72,8 @@ func ListMilestones(ctx *context.APIContext) {
 	for i := range milestones {
 		apiMilestones[i] = convert.ToAPIMilestone(milestones[i])
 	}
+
+	ctx.SetTotalCountHeader(total)
 	ctx.JSON(http.StatusOK, &apiMilestones)
 }