[follow-up] Filter board issues with persisted filters on the backend

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2912 adds extra flexibility to the Issue Boards. As an addition to persisting milestones on boards (which works as a default filtering scope for issues), now we persist assignee, weight and labels.

Currently, the frontend is sending these filtering params through URL params. But, ultimately, this logic should be handled by the backend.

This adds flexibility to the codebase, which can reuse the filtered board issues elsewhere. More details on https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2912#note_42817537.