Add system warnings

[Naapperas]

Due to a recent issue found with the scraper, there are some course units shared between 2 or more courses that do not appear on one of those courses.

There are ways to circumvent this (like selecting course units from a different course) but this is not ideal. Furthermore, users have no way of knowing when that is the case or if those issues apply to their specific case.

This issue proposes adding a way for system administrators to issue warnings that should be shown to users when they use the platform. These warnings could inform users of cases like the one depicted above.

Regarding implementation details, this issue needs to be separated between frontend and backend development since there is no direct way for us (NIAEFEUP) to issue those warnings directly to the users' browser clients.
For the frontend portion of this issue, we could establish a WebSocket connection to the backend that listens for these warnings and displays them upon receiving one.