Code cleanup grtlv 417 update learn to export footer and remove old footers

[FoxJamie16]

What

Adding the design system header to core/base.html as some sections (learn) don't use the domestic template. Removing unused footer styles and templates. Also added a very quick fix as the header menu javascript isn't working on prod, so added a block.super to fix.

Note - don't merge until I check with david that the core/base.html footer cache will still work as expected

Why

Make great more maintainable with cleaner styles

Tick or delete as appropriate:

Workflow

• Ticket exists in Jira https://uktrade.atlassian.net/browse/GRTLV-417
• Jira ticket has the correct status.
• A clear/description pull request messaged added.

Reviewing help

Housekeeping

• I have checked that my PR is using the latest package versions of: great-components, directory-constants, directory-healthcheck, directory-validators, directory-components, directory-api-client, directory-ch-client, django-staff-sso-client, directory-forms-api-client, directory-sso-api-client, sigauth

Security

• Frontend assets have been re-compiled
• Checked for potential security vulnerabilities
• Ensured any sensitive data is handled appropriately

Performance

• Evaluated the performance impact of the changes
• Ensured that changes do not negatively affect application scalability.

Merging

• This PR can be merged by reviewers. (If unticked, please leave for the author to merge)