Refactor toast messages

[bartacc]

As I said, toast is created as a reaction to user action: the user clicks the button, it calls API, and we have a "success" response, so we display a "success" toast. For this kind of message, we should be able to use toast function inside methods that handles this interactions/API call. For backend notifications, like "your lecture was accepted," which can happen without user interaction or even without the user being on the website, we should show some notification bell on the navbar that will display those messages ("System Zapisów" is a good example). A quick look through Django Docs shows that this is both, right? So, there is no good solution here.

If we can separate those two types of messages, that would be perfect, but maybe we should create a task for it later and leave it as it currently is.

Originally posted by @Arsenicro in #7 (comment)