Update remaining notebook imports to jupyter_server

[kevin-bates]

Updated a couple more instances of notebook import statements to reflect the jupyter_server location. The problem was because we use GatewayClient from the "running server" to determine whether a gateway server is configured or not and, if so, the configuration settings to use (URL, timeouts, etc.). Since this is a singleton and the reference is from the notebook package, yet jupyter_server is running, the notebook-relative singleton will not reflect the current configuration.

Fixes #1470

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