fix: Fix course-overviews sink not ordering blocks correctly

[bmtcril]

We've been using Modulestore's get_items, but even though it's often correct the ordering isn't guaranteed. The new method walks the course tree itself, then adds in the detached models separately.

Potential concerns:

This was already a pretty heavy operation, calling into Modulestore twice to walk all of the blocks may not be performant enough or cause issues in the LMS/CMS. It may be preferable to ignore detached blocks if we are not currently using them, but I wanted to preserve parity with what we've been doing unless/until there is an actual problem.

Closes: #61

Merge checklist: Check off if complete or not applicable:

• [x] Version bumped
• [x] Changelog record added
• [ ] Fixup commits are squashed away
• [x] Unit tests added/updated
• [ ] Manual testing instructions provided
• [x] Noted any: Concerns, dependencies, migration issues, deadlines, tickets

[github-actions[bot]]

Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
platform_plugin_aspects
__init__.py
platform_plugin_aspects/sinks
course_overview_sink.py
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