[FIX] calendar: Fix unicode reminders

Before this commit, if a reminder was created and the dates being
formatted contained unicode characters the whole thing would break
because python + unicode is hard.

Example: Japanese with Asia/Tokyo timezone and creating reminders
of 1 day, see support ticket 771674

addons/calendar/models/calendar.py

@@ -645,16 +645,30 @@ class Meeting(models.Model):
         date_deadline = fields.Datetime.context_timestamp(self.with_context(tz=timezone), fields.Datetime.from_string(stop))
 
         # convert into string the date and time, using user formats
-        date_str = date.strftime(format_date)
-        time_str = date.strftime(format_time)
+        date_str = date.strftime(format_date).decode('utf-8')
+        time_str = date.strftime(format_time).decode('utf-8')
 
         if zallday:
             display_time = _("AllDay , %s") % (date_str)
         elif zduration < 24:
             duration = date + timedelta(hours=zduration)
-            display_time = _("%s at (%s To %s) (%s)") % (date_str, time_str, duration.strftime(format_time), timezone)
+            duration_time = duration.strftime(format_time).decode('utf-8')
+            display_time = _(u"%s at (%s To %s) (%s)") % (
+                date_str,
+                time_str,
+                duration_time,
+                timezone,
+            )
         else:
-            display_time = _("%s at %s To\n %s at %s (%s)") % (date_str, time_str, date_deadline.strftime(format_date), date_deadline.strftime(format_time), timezone)
+            dd_date = date_deadline.strftime(format_date).decode('utf-8')
+            dd_time = date_deadline.strftime(format_time).decode('utf-8')
+            display_time = _(u"%s at %s To\n %s at %s (%s)") % (
+                date_str,
+                time_str,
+                dd_date,
+                dd_time,
+                timezone,
+            )
         return display_time
 
     def _get_duration(self, start, stop):