DEV: remove exec_sql and replace with mini_sql

Introduce new patterns for direct sql that are safe and fast.

MiniSql is not prone to memory bloat that can happen with direct PG usage.
It also has an extremely fast materializer and very a convenient API

- DB.exec(sql, *params) => runs sql returns row count
- DB.query(sql, *params) => runs sql returns usable objects (not a hash)
- DB.query_hash(sql, *params) => runs sql returns an array of hashes
- DB.query_single(sql, *params) => runs sql and returns a flat one dimensional array
- DB.build(sql) => returns a sql builder

See more at: https://github.com/discourse/mini_sql

lib/backup_restore/backup_restore.rb

@@ -76,7 +76,7 @@ module BackupRestore
   end
 
   def self.move_tables_between_schemas(source, destination)
-    User.exec_sql(move_tables_between_schemas_sql(source, destination))
+    DB.exec(move_tables_between_schemas_sql(source, destination))
   end
 
   def self.move_tables_between_schemas_sql(source, destination)
@@ -196,7 +196,7 @@ module BackupRestore
   end
 
   def self.backup_tables_count
-    User.exec_sql("SELECT COUNT(*) AS count FROM information_schema.tables WHERE table_schema = 'backup'")[0]['count'].to_i
+    DB.query_single("SELECT COUNT(*) AS count FROM information_schema.tables WHERE table_schema = 'backup'").first.to_i
   end
 
 end

lib/backup_restore/restorer.rb

@@ -379,14 +379,14 @@ module BackupRestore
 
       @db_was_changed = true
 
-      User.exec_sql(sql)
+      DB.exec(sql)
     end
 
     def migrate_database
       log "Migrating the database..."
       Discourse::Application.load_tasks
       ENV["VERSION"] = @current_version.to_s
-      User.exec_sql("SET search_path = public, pg_catalog;")
+      DB.exec("SET search_path = public, pg_catalog;")
       Rake::Task["db:migrate"].invoke
     end