[Refactor] Refactor Event.__as_tz() so it accepts a timezone

Until now it could only convert to the event’s timezone

calsocial/models.py

@@ -110,10 +110,12 @@ class Event(db.Model):
     #: The description of the event
     description = db.Column(db.UnicodeText())
 
-    def __as_tz(self, timestamp):
-        from pytz import timezone, utc
+    def __as_tz(self, timestamp, as_timezone=None):
+        from pytz import timezone, UTC
 
-        return utc.localize(timestamp).astimezone(timezone(self.time_zone))
+        utc_timestamp = UTC.localize(timestamp)
+
+        return utc_timestamp.astimezone(as_timezone or timezone(self.time_zone))
 
     @property
     def start_time_tz(self):