ARR commitment: fix paper_link validation

[melisabok]

Use a regex that check the link follow the format "https://openreview.net/forum?id=[note_id]"

No pdf link should be allowed and not extra parameters and no comma separated values.

https://github.com/openreview/openreview-py/blob/master/openreview/venue/process/submission_commitments_preprocess.py

Add a descriptive error message.