Add system checks to dashboard in admin UI (#15989)

app/lib/admin/system_check.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+class Admin::SystemCheck
+  ACTIVE_CHECKS = [
+    Admin::SystemCheck::DatabaseSchemaCheck,
+    Admin::SystemCheck::SidekiqProcessCheck,
+    Admin::SystemCheck::RulesCheck,
+  ].freeze
+
+  def self.perform
+    ACTIVE_CHECKS.each_with_object([]) do |klass, arr|
+      check = klass.new
+
+      if check.pass?
+        arr
+      else
+        arr << check.message
+      end
+    end
+  end
+end