Add input callback for tables.sql to prevent indexes being created
on the search index table.

Removes need for live hack for tables.sql in
https://gerrit.wikimedia.org/r/#/c/22466

> var_dump( strpos( 'CREATE UNIQUE INDEX /*i*/si_page ON /*_*/searchindex (si_page);', 'ON /*_*/searchindex' ) === false );
bool(false)

Change-Id: I9863978354cd50c18191671b08db158ed55f68c6