Add article_id and process test set template for semeval 2020 task 11…
… dataset

- `article_id` is needed to create the submission file for the task at https://propaganda.qcri.org/semeval2020-task11/
- The `technique classification` task provides the span indices in a template for the test set that is necessary to complete the task. This PR implements processing of that template for the dataset.
Thanks !
Now to fix the CI the only thing left is to add a dummy `test-task-tc-template.out` file inside the `dummy_data.zip` at `./datasets/sem_eval_2020_task_11/dummy/1.1.0`
It must contain the labels template for each dummy article of the test set included in `dummy_data.zip`

After that we should be good to merge this one :) @lhoestq Made the changes! The failure now seems to be unrelated to the changes. Any idea what's going on? This is a bug on master that we're investigating. You can ignore it