Exclude overviews from user data size

Fixes #261

Some internal functions from the Overviews module
have been moved to a separate file because they're now
used from Quaota function.

scripts-available/CDB_Quota.sql

@@ -35,7 +35,9 @@ BEGIN
   table_cat AS (
     SELECT
       table_name,
-      EXISTS(select * from raster_tables where o_table_name = table_name) AS is_overview,
+      EXISTS(select * from raster_tables where o_table_name = table_name)
+      OR table_name SIMILAR TO _CDB_OverviewTableDiscriminator() || '[\w\d]*'
+      AS is_overview,
       EXISTS(SELECT * FROM raster_tables WHERE r_table_name = table_name) AS is_raster
     FROM user_tables
   ),