Hide unreleased assessment attachments

[damianhxy]

Description

• Change logic to only display attachments whose corresponding assessments are released (note that we use a left_outer_join in order to include attachments that belong to the course, since they will have an assessment_id of nil)

Motivation and Context

Currently, released attachments of unreleased assessments are viewable to students (but not downloadable). While not a major issue, this is not desirable if e.g. the instructor wants to mark certain attachments as released ahead of time but does not want students to see their names.

Fixes #1527

How Has This Been Tested?

Created released and unreleased attachments for course, unreleased assessment (Homework 0), and released assessment (Homework 1). Student should only be able to see released attachments for course and Homework 1.

As instructor:

As student:

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have run rubocop for style check. If you haven't, run overcommit --install && overcommit --sign to use pre-commit hook for linting
• My change requires a change to the documentation, which is located at Autolab Docs
• I have updated the documentation accordingly, included in this PR

Other issues / help required

Unsure if syntax follows best practices.

[michellexliu]

lgtm!