Adds related course pages

[kevinhowbrook]

⚠️ based on #562

Description of Changes Made

This PR adds the related course page model, so courses can be related to one another.

How to Test

1. Check out this branch locally
2. Add a course landing page
3. Add 3 course pages
4. Relate some course pages to eachother
5. View front end of course page and you should see the related course pages (unstyled)

MR Checklist

• Add a description of your pull request and instructions for the reviewer to verify your work.
• If your pull request is for a specific ticket, link to it in the description.
• Stay on point and keep it small so the merge request can be easily reviewed.
• Consider adding unit tests, especially for bug fixes. If you don't, tell us why.
• Tests and linting passes.
• Consider updating documentation. If you don't, tell us why.
• If relevant, list the environments / browsers in which you tested your changes.

[William-Blackie]

Looks good, one suggestion :)