[C2] Glossary tool fixes

[ywarnier]

The new glossary tool (see #4768 ) barely works. It needs a series of fixes:

• Creating term works for me
• Saving term (when editing) does not work (Could not create glossary term)
• CSV export works for me
• PDF export does not work
• XLS export does not work (creates file but corrupted) (should be xlsx, by the way)
• CSV import does not work ("Could not import terms")
• XLS (or XLSX) import does not work (same as CSV)
• table view works for me
• Student view button does not react to clicks