[STORY] Event messaging system (pub-sub, etc.)

[ValerianClerc]

We're starting to have a lot of events that require contacting other microservices. Currently, we're writing custom endpoints for this. Eventually we might want to set up an event system to handle these, rather than making direct calls.

Todo:

• Research whether we want to use pub-sub, or continue direct calls to a defined interface (gRPC-like)

Interactions that would require pub-sub (please keep this up to date):

• resetLastSubmission in the users' microservice when a new problemset is created
• deleting problemset => also delete ID in user model
• the mailing microservice?