[A11y] Improve customer account pages (#4654)

src/pretix/base/models/memberships.py

@@ -159,10 +159,24 @@ class Membership(models.Model):
         de = date_format(self.date_end, 'SHORT_DATE_FORMAT')
         return f'{self.membership_type.name}: {self.attendee_name} ({ds} – {de})'
 
+    @property
+    def percentage_used(self):
+        if self.membership_type.max_usages and self.usages:
+            return int(self.usages / self.membership_type.max_usages * 100)
+        return 0
+
     @property
     def attendee_name(self):
         return build_name(self.attendee_name_parts, fallback_scheme=lambda: self.customer.organizer.settings.name_scheme)
 
+    @property
+    def expired(self):
+        return time_machine_now() > self.date_end
+
+    @property
+    def not_yet_valid(self):
+        return time_machine_now() < self.date_start
+
     def is_valid(self, ev=None, ticket_valid_from=None, valid_from_not_chosen=False):
         if valid_from_not_chosen:
             return not self.canceled and self.date_end >= time_machine_now()