feat(backend): Generic authentication checks.

Added support for deciding based on any predicate about the current AuthenticationInfo whether or not a request will be accepted.

backend/src/middleware/auth/authentication-info.ts

@@ -0,0 +1,11 @@
+/**
+ * Object with information about the user who is currently logged in.
+ */
+export type AuthenticationInfo = {
+    accountType: "student" | "teacher",
+    username: string,
+    name?: string,
+    firstName?: string,
+    lastName?: string,
+    email?: string
+};