Ability to export full confusion matrix with over 50 labels

[StephenChan]

See: https://groups.google.com/g/coralnet-users/c/zG7oRReu1Pk

After the 50 most common labels [in the validation set], the confusion matrix always groups the rest of the labels as 'Other'.

I believe we cut off the matrix at 50 in order to make the matrix fit reasonably on the backend page. However, for CSV exporting, we don't need to limit the matrix size.

The simplest thing to do is to just change the export behavior to always use the full matrix. Or, we could have an option on exporting the full matrix vs. exporting top 50 + 'Other' (the option would only be relevant if there are over 50 labels).