Optimise SQL queries when computing balance

This avoids creating thousands of small SQL queries when computing the
balance of users.  This significantly improves the performance of
displaying the main page of a project, since the balance of users is
displayed there:

    Before this commit: 4004 SQL queries, 19793 ms elapsed time, 19753 ms CPU time, 2094 ms SQL time
    After this commit:    12 SQL queries,  3688 ms elapsed time,  3753 ms CPU time,   50 ms SQL time
    Measured request:   display the sidebar with the balance of all users for the project (without displaying the list of bills)

This commit also greatly improves the performance of the "settle bills" page:

    Before this commit: 8006 SQL queries, 39167 ms elapsed time, 39600 ms CPU time, 4141 ms SQL time
    After this commit:    22 SQL queries,  7144 ms elapsed time,  7283 ms CPU time,   96 ms SQL time
    Measured request:   display the "Settle bills" page

Test setup to measure performance improvement:

- 5 users with various weights
- 1000 bills, each paid by a random user, each involving all 5 users
- laptop with Celeron N2830@2.16 GHz, SSD Samsung 850 EVO
- sqlite database on SSD, using sqlite 3.15.2
- python 2.7.13
- Flask-DebugToolbar 0.10.0 (to count SQL queries and loading time)

Performance measurements (using Flask-DebugToolbar on the second request,
to avoid measuring cold-cache performance):

- number of SQL queries
- elapsed time (from request to response)
- total CPU time consumed by the server handling the request
- total time spent on SQL queries (as reported by SQLAlchemy)

budget/models.py

@@ -37,7 +37,7 @@ class Project(db.Model):
         # for each person
         for person in self.members:
             # get the list of bills he has to pay
-            bills = Bill.query.filter(Bill.owers.contains(person))
+            bills = Bill.query.options(orm.subqueryload(Bill.owers)).filter(Bill.owers.contains(person))
             for bill in bills.all():
                 if person != bill.payer:
                     share = bill.pay_each() * person.weight