Enforce a maximum number of file uploads and expose the limit to the front-end

In #2327, we're enforcing a maximum of 8 file uploads to a single notebook. We should enforce this on the server-side and expose the limit (maybe 8, maybe some other number) to the front-end.

Start by reading issue #2327 and then locate the notebook file-upload paths on both the server and front-end. Done means uploads beyond the configured maximum are rejected server-side and the front-end receives that limit rather than assuming a fixed value.

Written by the indexing model from the issue text.