Refactor `submissions_by_grader_for_index` and related functions to use piping

[GabrielCWT]

In order to keep styling consistent throughout the backend, piping should be used in the following functions

• [ ] submissions_by_grader_for_index
• [ ] build_user_filter
• [ ] build_assessment_filter
• [ ] build_assessment_config_filter
• [ ] build_submission_filter
• [ ] build_course_registration_filter

[GabrielCWT]

As an add on, I feel that it could be possible that after the refactor submissions_by_grader_for_index's count query can go before the submissions query such that there is less repeated code.

query = big_query_here

count = query |> select([...], count(s.id)) |> Repo.one()
submissions = query |> limit(...) |> offset(...) |> select(...) |> Repo.all()