Fix database permission check for triggers on Amazon RDS

On some instances of Amazon RDS, running a SELECT against
`'information_schema.role_table_grants'` fails with a `permission denied
for relation pg_authid` error. This happens because `role_table_grants`
is a PostgreSQL view that ultimately accesses the `pg_authid` table.

The PostgreSQL system function `has_table_privileges` doesn't actually
need a FROM clause. We can simplify the query to just call:

SELECT has_table_privilege('projects', 'TRIGGER')

Closes https://gitlab.com/gitlab-org/gitlab/issues/199468

changelogs/unreleased/sh-fix-trigger-check-rds.yml

+---
+title: Fix database permission check for triggers on Amazon RDS
+merge_request: 24035
+author:
+type: fixed

lib/gitlab/database/grant.rb

@@ -3,23 +3,18 @@
 module Gitlab
   module Database
     # Model that can be used for querying permissions of a SQL user.
-    class Grant < ActiveRecord::Base
-      include FromUnion
-
-      self.table_name = 'information_schema.role_table_grants'
-
+    class Grant
       # Returns true if the current user can create and execute triggers on the
       # given table.
       def self.create_and_execute_trigger?(table)
         # We _must not_ use quote_table_name as this will produce double
         # quotes on PostgreSQL and for "has_table_privilege" we need single
         # quotes.
+        connection = ActiveRecord::Base.connection
         quoted_table = connection.quote(table)
 
         begin
-          from(nil)
-            .pluck(Arel.sql("has_table_privilege(#{quoted_table}, 'TRIGGER')"))
-            .first
+          connection.select_one("SELECT has_table_privilege(#{quoted_table}, 'TRIGGER')").present?
         rescue ActiveRecord::StatementInvalid
           # This error is raised when using a non-existing table name. In this
           # case we just want to return false as a user technically can't