Restyle error show/hide buttons

[codemacabre]

Summary

• Refactor and restyle the 'Show/Hide Errors' buttons
• Ensure screenreaders announce buttons more accessibly (i.e. "Show [error type] errors")

Fixes: https://github.com/Open-Telecoms-Data/cove-ofds/issues/65

Verify

1. Upload JSON which contains errors
2. Once processed, scroll to the 'Structure and Format' panel. Each error type should have:
   • Number of errors beside error type
   • A button with a chevron-down icon
3. Activating the down-chevron button should:
   • Expand the error table
   • Change the chevron-down icon to a chevron-up icon

A 0.5s delay prevents rapidly expanding and collapsing the table, to prevent UI problems.

[odscjames]

Thanks!

JS in cove_ofds/templates/cove_ofds/jsonschema_validation_panel.html - would be interesting to have a chat later about where is a good place for JS to live. Feels like several things to balance - page performance, maintainability ... No idea what right answer is!