The database upgrade code looks for SCMTool-related state to migrate

over to SCMTool IDs, and one of the places where it looks is

IntegrationConfig.
The problem is, this assumes IntegrationConfig was installed in the

database prior to upgrade. That breaks upgrades from Review Board 2.5

and older.
This change fixes up that assumption by first querying a list of all

known tables in the database as the first pre-upgrade step. Any code

that's sensitive to the presence of tables (ones that haven't been

around since 1.0) now checks for presence before doing anything that

would touch the database.
Existing code that used DatabaseError to determine if tables existed

have been updated to use this mechanism. This leaves DatabaseError

free to bubble up, so we can catch other types of errors.

Tested upgrading from 2.5, 3.0, 4.0, and 5.0 databases using both

./reviewboard/manage.py and rb-site upgrade.
Tested installing a new database.