Add course pages FE

[siimonevans]

⚠️ Based on #562

Description of Changes Made

This PR builds on Kev's BE work, adding FE for the course landing/detail pages.

No tickets for this feature.

Designs here - note we've deviated in some places in order to re-use existing styles, e.g 'What you’ll learn' section.

How to Test

See #562 for instructions on which pages to add and further detail. To review, the reviewer will need to manually create and populate these pages in Wagtail - we're not bothering with PL data given its existing state.

In one or two places I've had to override existing CSS specificity, or pulled in code from the Careers site - it wasn't deemed worth the effort to bring these up to scratch for this codebase given the imminent redesign project.

Screenshots

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]

Hey Simon, looks good! Mostly nitpicks.