Fix joining Shift+ShiftEntry, unhack volunteer schedule_ical

https://docs.sqlalchemy.org/en/14/orm/mapped_sql_expr.html leads me to
believe this was just a bug in the definition of current_count.

apps/volunteer/schedule.py

@@ -6,7 +6,7 @@
 from flask import current_app as app
 from collections import defaultdict
 from flask_login import current_user
-from sqlalchemy.orm import joinedload, defer
+from sqlalchemy.orm import joinedload
 
 from main import db
 from models import event_year
@@ -121,7 +121,6 @@ def schedule_ical():
             .select_from(ShiftEntry)
             .join(Shift.entries.and_(ShiftEntry.user == user))
             .options(
-                defer(Shift.current_count),  # the subquery for that column breaks this query
                 joinedload(Shift.venue),
                 joinedload(Shift.role)
             )).all()

models/volunteer/shift.py

@@ -71,7 +71,7 @@ class Shift(BaseModel):
     proposal = db.relationship("Proposal", backref="shift")
 
     current_count = db.column_property(
-        select([func.count(ShiftEntry.shift_id)]).where(ShiftEntry.shift_id == id).scalar_subquery()  # type: ignore[attr-defined]
+        select([func.count(ShiftEntry.shift_id)]).where(ShiftEntry.shift_id == id).correlate_except(ShiftEntry).scalar_subquery()  # type: ignore[attr-defined]
     )
 
     duration = db.column_property(end - start)