Add a button to export model's training data

Here we are going to test that:

• Multiple training task executions result in multiple training files generated.
• These file contents can be combined into 1 and downloaded

1. You need 1 course with a past start date and end date and with at least 1 student
2. Students at risk of dropping out models should be enabled and should have a time splitting method defined
3. Moodle will only use courses for training if they have a minimum of activity (more than 10 logs per student in a course) You can generate all these logs or just edit lib/classes/analytics/target/course_dropout.php is_valid_analysable and make it return true

4. php admin/tool/task/cli/schedule_task.php --execute=\\tool_analytics\\task\\train_models

5. Go to admin/tool/analytics/index.php and Actions > Export "Students at risk of dropping out" model
6. Open the downloaded file and note the number of rows
7. Enrol another 2 students to that course

8. php admin/tool/task/cli/schedule_task.php --execute=\\tool_analytics\\task\\train_models

9. Go to admin/tool/analytics/index.php and Actions > Export "Students at risk of dropping out" model
10. Open the downloaded file and note the number of rows
11. It SHOULD be higher than the previous number of rows

12. php admin/tool/analytics/cli/evaluate_model.php --modelid=XXX (where XXX is 'Students at risk of dropping out' id)

13. php admin/tool/task/cli/schedule_task.php --execute=\\tool_analytics\\task\\train_models

14. Go to admin/tool/analytics/index.php and Actions > Export "Students at risk of dropping out" model
15. Open the downloaded file, the number of rows SHOULD be equal to the previous one but higher than the former one (here we are testing that evaluation files are not merged into the exported file)