Better workflow for drag & dropping excel files into Iodide

Dragging and dropping an excel file into an iodide notebook currently works (e.g. this notebook was created that way: https://alpha.iodide.io/notebooks/3662/) however there are no clear instructions for what to do with the result. For this particular case, we should probably pre-populate a notebook with the right incantations for loading it with pandas.

First step is to figure out how to load excel data into pandas at all :) /cc @deborahanneharris

Start with the linked notebook example and investigate how a dragged Excel file is represented in Iodide and how pandas can load it. Determine the existing notebook-generation entry point before deciding how to pre-populate the appropriate loading instructions. Done means a dragged Excel file produces a notebook with clear, working pandas loading guidance.

Written by the indexing model from the issue text.