Add tests for maximum database queries

[timobrembeck]

Motivation

At the moment, we only check the API result, not the time it takes to generate the correct API result.
It's still part of the manual development/reviewing process to detect changes which drastically decrease API performance.
Usually, database queries are the bottleneck for your API views, especially when some related field is not prefetched etc...

Proposed Solution

Add tests for all API views which define a reasonable maximum of database queries (maybe 10-20, depending on the view) and assert they are not exceeded.
This can be achieved by django_assert_max_num_queries:

def test_max_queries(django_assert_max_num_queries):
    with django_assert_max_num_queries(2):
        Item.objects.create('foo')
        Item.objects.create('bar')