Introduce db table to store users statistics

The users statistics will be stored in the database to improve
performance.

app/models/users_statistics.rb

+# frozen_string_literal: true
+
+class UsersStatistics < ApplicationRecord
+  STATISTICS_NAMES = [
+    :without_groups_and_projects,
+    :with_highest_role_guest,
+    :with_highest_role_reporter,
+    :with_highest_role_developer,
+    :with_highest_role_maintainer,
+    :with_highest_role_owner,
+    :bots,
+    :blocked
+  ].freeze
+end

changelogs/unreleased/create_users_statistics.yml

+---
+title: Introduce db table to store users statistics
+merge_request: 26261
+author:
+type: added

db/migrate/20200221100514_create_users_statistics.rb

+# frozen_string_literal: true
+
+class CreateUsersStatistics < ActiveRecord::Migration[6.0]
+  DOWNTIME = false
+
+  def change
+    create_table :users_statistics do |t|
+      t.timestamps_with_timezone null: false
+      t.integer :without_groups_and_projects, null: false, default: 0
+      t.integer :with_highest_role_guest, null: false, default: 0
+      t.integer :with_highest_role_reporter, null: false, default: 0
+      t.integer :with_highest_role_developer, null: false, default: 0
+      t.integer :with_highest_role_maintainer, null: false, default: 0
+      t.integer :with_highest_role_owner, null: false, default: 0
+      t.integer :bots, null: false, default: 0
+      t.integer :blocked, null: false, default: 0
+    end
+  end
+end

db/schema.rb

@@ -4420,6 +4420,19 @@ ActiveRecord::Schema.define(version: 2020_03_09_105539) do
     t.index ["user_id", "project_id"], name: "index_users_star_projects_on_user_id_and_project_id", unique: true
   end
 
+  create_table "users_statistics", force: :cascade do |t|
+    t.datetime_with_timezone "created_at", null: false
+    t.datetime_with_timezone "updated_at", null: false
+    t.integer "without_groups_and_projects", default: 0, null: false
+    t.integer "with_highest_role_guest", default: 0, null: false
+    t.integer "with_highest_role_reporter", default: 0, null: false
+    t.integer "with_highest_role_developer", default: 0, null: false
+    t.integer "with_highest_role_maintainer", default: 0, null: false
+    t.integer "with_highest_role_owner", default: 0, null: false
+    t.integer "bots", default: 0, null: false
+    t.integer "blocked", default: 0, null: false
+  end
+
   create_table "vulnerabilities", force: :cascade do |t|
     t.bigint "milestone_id"
     t.bigint "epic_id"