[Refactor] Rename the app module to calsocial

Just for clarity

calsocial/calendar_system/__init__.py

@@ -0,0 +1,15 @@
+class CalendarSystem:
+    __has_months__ = False
+    __has_weeks__ = False
+
+    @property
+    def day_names(self):
+        raise NotImplementedError()
+
+    @property
+    def month(self):
+        raise NotImplementedError()
+
+    @property
+    def days(self):
+        raise NotImplementedError()