Add reminder to contribute for previous exchanges

We should do our best to encourage students to contribute to the platform.

Under the following constraints:

• There is no way for us to know for sure that a student has contributed for each of his or her real exchanges.
• We only know about the student exchanges that he or she has given authorization to the DSI to share with REX-DRI.

So creating a smart / deterministic reminder might be too complicated.

Sending reminder by emails, is out of the scope of this project.

One possibility would be recurring notification, that show up once every few weeks / months after the last time the person has clicked on the popup.