Closes #80: Always use utc datetime to prevent inconsistent dates

tracim/tracim/model/auth.py

     group_id = Column(Integer, Sequence('seq__groups__group_id'), autoincrement=True, primary_key=True)
     group_name = Column(Unicode(16), unique=True, nullable=False)
     display_name = Column(Unicode(255))
-    created = Column(DateTime, default=datetime.now)
+    created = Column(DateTime, default=datetime.utcnow)
 
     users = relationship('User', secondary=user_group_table, backref='groups')
 
     email = Column(Unicode(255), unique=True, nullable=False)
     display_name = Column(Unicode(255))
     _password = Column('password', Unicode(128))
-    created = Column(DateTime, default=datetime.now)
+    created = Column(DateTime, default=datetime.utcnow)
     is_active = Column(Boolean, default=True, nullable=False)
     imported_from = Column(Unicode(32), nullable=True)
     _webdav_left_digest_response_hash = Column('webdav_left_digest_response_hash', Unicode(128))

tracim/tracim/model/data.py

             column_value = getattr(revision, column_name)
             setattr(new_rev, column_name, column_value)
 
-        new_rev.updated = datetime.now()
+        new_rev.updated = datetime.utcnow()
 
         return new_rev
 
 
     def created_as_delta(self, delta_from_datetime:datetime=None):
         if not delta_from_datetime:
-            delta_from_datetime = datetime.now()
+            delta_from_datetime = datetime.utcnow()
 
         return format_timedelta(delta_from_datetime - self.created,
                                 locale=tg.i18n.get_lang()[0])
     def datetime_as_delta(self, datetime_object,
                           delta_from_datetime:datetime=None):
         if not delta_from_datetime:
-            delta_from_datetime = datetime.now()
+            delta_from_datetime = datetime.utcnow()
         return format_timedelta(delta_from_datetime - datetime_object,
                                 locale=tg.i18n.get_lang()[0])
 
 
     def created_as_delta(self, delta_from_datetime:datetime=None):
         if not delta_from_datetime:
-            delta_from_datetime = datetime.now()
+            delta_from_datetime = datetime.utcnow()
         return format_timedelta(delta_from_datetime - self.created,
                                 locale=tg.i18n.get_lang()[0])
 
         aff = ''
 
         if not delta_from_datetime:
-            delta_from_datetime = datetime.now()
+            delta_from_datetime = datetime.utcnow()
 
         delta = delta_from_datetime - self.created
         

tracim/tracim/model/serializers.py

     last_activity_date = content.get_last_activity_date()
     last_activity_date_formatted = format_datetime(last_activity_date,
                                                    locale=tg.i18n.get_lang()[0])
-    last_activity_label = format_timedelta(datetime.now() - last_activity_date,
-                                           locale=tg.i18n.get_lang()[0])
+    last_activity_label = format_timedelta(
+        datetime.utcnow() - last_activity_date,
+        locale=tg.i18n.get_lang()[0],
+    )
     last_activity_label = last_activity_label.replace(' ', '\u00A0') # espace insécable
 
     return DictLikeClass(
     last_activity_date = content.get_last_activity_date()
     last_activity_date_formatted = format_datetime(last_activity_date,
                                                    locale=tg.i18n.get_lang()[0])
-    last_activity_label = format_timedelta(datetime.now() - last_activity_date,
+    last_activity_label = format_timedelta(datetime.utcnow() - last_activity_date,
                                            locale=tg.i18n.get_lang()[0])
     last_activity_label = last_activity_label.replace(' ', '\u00A0') # espace insécable