(sessions): Add study_time_since function.

New `study_time_since` database function from session history.
Add `Lion.timezone`.
Add `Lion.day_start`.
Add `Lion.studied_today`.
Made `Table.queries` an instance variable.
Renamed the session channel types.

bot/data/interfaces.py

@@ -45,10 +45,10 @@ class Table:
     Intended to be subclassed to provide more derivative access for specific tables.
     """
     conn = conn
-    queries = DotDict()
 
     def __init__(self, name, attach_as=None):
         self.name = name
+        self.queries = DotDict()
         tables[attach_as or name] = self
 
     @_connection_guard