when importing excel spreadsheets, give an alert if they're odd

[rubenarslan]

three things recently bit us:

• [x] apparently different cells can contain some characters formatted as NFC and some as NFD: https://github.com/rubenarslan/formr.org/issues/131
• [ ] fused cells
• [ ] excess columns, rows (this leads to out-of-memory problems), people sometimes creates millions of blank columns by accident.

[mpw-wwu]

I think @cyriltata fixed checkbox three already with commit d965ac6c7. Can be checked.