Don't sort todos when counting them

Grouping isn't as lax in PostgreSQL as it is in MySQL or SQLite. All
sort fields also need to be in the GROUP BY, or be aggregated. The order
isn't relevant when counting, so simply don't order in that case.

Fix #1336

app/models/user.rb

@@ -81,7 +81,11 @@ class User < ActiveRecord::Base
             end
   has_many :todos,
            :order => 'todos.completed_at DESC, todos.created_at DESC',
-           :dependent => :delete_all
+           :dependent => :delete_all do
+              def count_by_group(g)
+                except(:order).group(g).count
+              end
+           end
   has_many :recurring_todos,
            :order => 'recurring_todos.completed_at DESC, recurring_todos.created_at DESC',
            :dependent => :delete_all