Support Solr core used in Sanofi's Oracle branch

Clean old cores with `make -C solr clean_cores`
and start the Oracle core with `ORACLE=1 make -C solr start`

Maybe we should have separate directories for Oracle/PostgreSQL, so
that we don't have to clean the cores when changing RDBMSs.
Sanofi branch requires a few config overrides

Add a Config-extra.php sample file for this purpose.