Update course section expander to nhsuk version

[frank-hee]

JIRA link

TD-5734

Description

Added mustache files to theme to implement nhsuk expander code.
Moodle version had button to expand next to text to link to the section. Nhsuk version uses button and text to expand/collapse
Added a link to sections after the expander header in line with nhsuk standard for expander buttons and titles.
some css updates.

Screenshots

While there are Axe and Wave errors relating to landmarks and roles, these pre-exist this PR and are not related to these changes.

Developer checks

(Leave tasks unticked if they haven't been appropriate for your ticket.)

I have:

• Run the formatter and made sure there are no IDE errors
• Written appropriate unit tests for the changes
• Manually tested my work with and without JavaScript
• Tested any Views or partials created or changed with Wave Chrome plugin and addressed any valid accessibility issues
• Updated/added documentation in Confluence and/or GitHub Readme. List of documentation links added/changed:
  • doc_1_here
• Updated my Jira ticket with information about other parts of the system that were touched as part of the MR and have to be sanity tested to ensure nothing is broken
• Scanned over my pull request in GitHub and addressed any warnings from the GitHub Build and Test checks.