[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/repo.go

@@ -230,8 +230,7 @@ func Search(ctx *context.APIContext) {
 	}
 
 	ctx.SetLinkHeader(int(count), opts.PageSize)
-	ctx.Header().Set("X-Total-Count", fmt.Sprintf("%d", count))
-	ctx.Header().Set("Access-Control-Expose-Headers", "X-Total-Count, Link")
+	ctx.SetTotalCountHeader(count)
 	ctx.JSON(http.StatusOK, api.SearchResults{
 		OK:   true,
 		Data: results,