[core] Truncate SEARCH Column in Document2 for Oracle database

Truncate Document2 SEARCH length to 2000

Searching saved documents in Oracle returns following error:
DatabaseError: ORA-06502: PL/SQL: numeric or value error: character string buffer too small

This is caused by DBMS_LOB.SUBSTR(%s, 4000) in Django framework django/db/backends/oracle/base.py
Django has a ticket for this issue but unfixed: https://code.djangoproject.com/ticket/11580.
Buffer size 4000 limit the length of field equals or less than 2000 characters.

For performance reasons and to avoid searching in huge fields, we also truncate to a max length 2000