Optimize jobs-list endpoint with push_id filter

[vrigal]

This endpoint cannot be simplified much, as built with a viewset + django-filters.

The majority of requests use the push_id__in filter, which performs a single lookup query (50+% of the requests compare with a single push ID using the __in clause).
However, 10% of the overall queries uses the push_id filter directly (you can see thoses request when loading the app main page), which cause an extra query to fetch the push entry before listing related jobs.

This implementation cuts off this extra query, improving perf by approx. 12% (389ms → 341ms).
On the other side, the API will return an empty jobs list when filtering with a wrong ID, instead of returning an explicit message (that was "push_id": ["Select a valid choice. That choice is not one of the available choices."]).