Practise only on a subset of features

[camilstaps]

A user* has suggested the following improvement:

The ability to select to practise identifying only stems, or only tenses, or tenses and stems but without person, gender and number. This would help our students to focus on their specific areas for improvement.

This would have to be customizable (i.e., the user should be able to select the features to train on).

_{* See email March 14th, 2022}