batches: fix wrong read-only namespace selector value
Created by: courier-new
Closes https://github.com/sourcegraph/sourcegraph/issues/40158.
Before we had a read-only version of the configuration page, the only options available in the namespace dropdown selector are those namespaces in which the current user is able to create batch changes: namely, their own user namespace and any org namespaces they are part of. But if a user is viewing another user's batch change and the read-only configuration page, that namespace isn't going to be one they technically have access to, so we need to populate it differently. This just fixes that.
I also tightened up some component prop type definitions while I was here to eliminate impossible combinations. The type definitions got grosser as a result, of course, but I think with the effect of better type feedback is still net positive.
Test plan
Manually tested.
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