Disable triggers during migrations

[BrentWheeldon]

The audit log insertions slow AR migrations down a lot (>30mins to less than 10 seconds).

[tmikoss]

There's a PgAuditLog::Function::DISABLED_USER constant - setting current user to that should cause the trigger to exit immediately, maybe that helps.

[amarshall]

We’d prefer to never implicitly disable triggers for fear that auditing would be unknowingly turned off, posing increased risk. Instead you can use PgAuditLog::Triggers.without_triggers { do_stuff } to turn off triggers for a block (which essentially does what @tmikoss suggested under-the-hood).