Prevent re-running of code cells

[parente]

Current implementation prevents arbitrary code from the browser to reach the kernel. However, the browser can instruct the proxy to resend one of the code cells from a notebook multiple times. A bad actor might take advantage of this to mess with state on the backend if the notebook code is not written to account for multiple runs (e.g., incrementing a counter too many times).

Still, this is not a serious bug: the code already has to run once on the first load of the dashboard.