Optimise SQL queries when displaying bills for a project

By defaut, SQLAlchemy uses lazy loading, which means that displaying n
bills will generate around n queries (to get the list of owers of each
bill).  Pre-load the list of owers to drastically decrease the number of
SQL queries.

Before this commit: 1004 SQL queries, 7535 ms elapsed time, 7536 ms CPU time, 530 ms SQL time

After this commit: 5 SQL queries, 3342 ms elapsed time, 3393 ms CPU time, 15 ms SQL time

Measured request: display the list of all bills for the project (without displaying the sidebar with balances)

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 with 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)

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