[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/collaborators.go

@@ -47,15 +47,24 @@ func ListCollaborators(ctx *context.APIContext) {
 	//   "200":
 	//     "$ref": "#/responses/UserList"
 
+	count, err := ctx.Repo.Repository.CountCollaborators()
+	if err != nil {
+		ctx.InternalServerError(err)
+		return
+	}
+
 	collaborators, err := ctx.Repo.Repository.GetCollaborators(utils.GetListOptions(ctx))
 	if err != nil {
 		ctx.Error(http.StatusInternalServerError, "ListCollaborators", err)
 		return
 	}
+
 	users := make([]*api.User, len(collaborators))
 	for i, collaborator := range collaborators {
 		users[i] = convert.ToUser(collaborator.User, ctx.User)
 	}
+
+	ctx.SetTotalCountHeader(count)
 	ctx.JSON(http.StatusOK, users)
 }