housekeep(fastapi): rework aurweb.db session API

Changes:
-------
- Add aurweb.db.get_session()
    - Returns aurweb.db's global `session` instance
    - Provides us a way to change the implementation of the session
      instance without interrupting user code.
- Use aurweb.db.get_session() in session API methods
- Add docstrings to session API methods
- Refactor aurweb.db.delete
    - Normalize aurweb.db.delete to an alias of session.delete
- Refresh instances in places we depend on their non-PK columns
  being up to date.

Signed-off-by: Kevin Morris <kevr@0cost.org>

aurweb/models/user.py

@@ -146,7 +146,7 @@ class User(Base):
         self.authenticated = False
         if self.session:
             with db.begin():
-                db.session.delete(self.session)
+                db.delete(self.session)
 
     def is_trusted_user(self):
         return self.AccountType.ID in {