Venue: add opt-in rejected funtionality

[celestemartinez]

Some venues like NeurIPS and EMNLP let rejected papers opt-in to be made public. The workflow looks like this:

• create opt-in invitations for rejected papers for authors to select if they want to make paper public or keep private
• in the post decision stage, we can add an option to release opt-in rejected papers. Release will be exactly the same as for accepted papers. Only release if at least one author agreed to make public and no authors asked to keep private

Note: this means that if reviews are released to the public after releasing opt-in rejected papers, reviews of these rejected papers will also be released