perf(captcha): simplify count() query for user ids

Using .count() isn't great as it runs a count query on a subquery which
selects all fields in the Users table. This rewrites it into a simple
SELECT count(ID) from USers query.

aurweb/captcha.py

@@ -3,6 +3,7 @@
 import hashlib
 
 from jinja2 import pass_context
+from sqlalchemy import func
 
 from aurweb.db import query
 from aurweb.models import User
@@ -11,7 +12,8 @@ from aurweb.templates import register_filter
 
 def get_captcha_salts():
     """Produce salts based on the current user count."""
-    count = query(User).count()
+    count = query(func.count(User.ID)).scalar()
+
     salts = []
     for i in range(0, 6):
         salts.append(f"aurweb-{count - i}")