[Refactor] Change GregorianCalendar.day_events() to a static method

It takes a date as its parameter, and doesn’t use the class’ timestamp, so there’s no point making
it an instance method.

calsocial/calendar_system/gregorian.py

@@ -142,7 +142,8 @@ class GregorianCalendar(CalendarSystem):
 
         return now >= month_start_timestamp and now < month_end_timestamp
 
-    def day_events(self, date, user=None):
+    @staticmethod
+    def day_events(date, user=None):
         from ..models import Event
 
         events = Event.query