Bug 31051: (QA follow-up) Use a single query and avoid duplicated sums

Bug 31051: (QA follow-up) Use a single query and avoid duplicated sums

This patch makes the `get_savings` method use a single query to
calculate the sum of the replacement prices. This way we save one query
per item and just rely on the DB features.

It has a side effect: we are not summing items twice.

Added tests for the 'itenumber is null' pathological but common use
case (specially in old_issues), as mentioned by Lucas. Handling for this
is added (grep filtering out undefined ones) and also in the return, for
the empty case, with // 0.

To test:
1. Apply this patch
2. Run:
   qa -c 6 --run-tests
=> SUCCESS: All good
3. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>