Optimise SQL queries

[zorun]

A project with a few hundreds bills loads quite slowly: rendering the list_bills view takes several seconds.

Debugging with the Flask toolbar, it seems that the number of SQL queries issued by list_bills is very high: number of bills * number of owers. For instance, with 100 bills and an average of 5 owers for each bill, there will be 500 SQL queries.

See also #156 for some initial thoughts on this issue

[zorun]

Most of the SQL queries are actually done by the sidebar, presumably to compute the balance. The list of bills itself generates approximately one query per bill (which is still too much).