[FEATURE] Allow opening .csv and .xlsx files as spreadsheets

[mferrera]

These files are used frequently in different kinds of analyses and configuration and may be more frequently edited than most configuration files. There are node packages to read excel files.

A few candidates:

• Handsontable -- commercial, non-permissive license
• Ag-grid -- MIT license but may paywall best features in commercial license
• ReactDataGrid -- MIT license but may paywall best features in commercial license
• ReactGrid -- MIT license but appears inactive

There might be other, better options, but this seems to be a very commercialized product space.

[rubenthoms]

Implemented by #82.