[TASK] Add tt_content index for CType

This is beneficial for very large tt_content
tables to speed up the number of used elements
lookup.

Author: nhovratov

ext_tables.sql

@@ -0,0 +1,4 @@
+-- Add index for CType to speed up number of used elements lookup --
+CREATE TABLE tt_content (
+	KEY CType (CType),
+);