Get the count of the all system users at endpoint /users/stats (#8847)

* Get the count of the all system users at endpoint /users/stats

* Added GetTotalUsersStats test in api4

* Changed pluralization and added the test back.

model/users_stats.go

@@ -0,0 +1,24 @@
+// Copyright (c) 2016-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package model
+
+import (
+	"encoding/json"
+	"io"
+)
+
+type UsersStats struct {
+	TotalUsersCount int64 `json:"total_users_count"`
+}
+
+func (o *UsersStats) ToJson() string {
+	b, _ := json.Marshal(o)
+	return string(b)
+}
+
+func UsersStatsFromJson(data io.Reader) *UsersStats {
+	var o *UsersStats
+	json.NewDecoder(data).Decode(&o)
+	return o
+}