batches: restore global nav item on sourcegraph.com

Created by: courier-new

Whoops #23052 accidentally removed the global Batch Changes navigation item on sourcegraph.com. 🤦‍♀️

Since this is pretty much the only exception to the rules of disabling/hiding/blocking anything batch-changes-related on the frontend if batch changes aren't enabled on the instance, I feel like it's fine to just make + state the exception from the frontend code as opposed to building another variable into that JSContext.