TD-4916 nhsuk override consolidation

[frank-hee]

JIRA link

TD-4916

Description

.nhsuk items moved from index.scss and layout.scss into nhsuk.scss
_layout.cshtml headers updated to reflect current design system and remove need for several overrides

Screenshots

---

Developer checks

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

I have:

• Run the IDE auto formatter on all files I’ve worked on and made sure there are no IDE errors relating to them
• Written or updated tests for the changes (accessibility ui tests for views, tests for controller, data services, services, view models created or modified) and made sure all tests are passing
• Manually tested my work with and without JavaScript (adding notes where functionality requires JavaScript)
• Tested any Views or partials created or changed with Wave Chrome plugin. Addressed any valid accessibility issues and documented any invalid errors
• Updated my Jira ticket with testing notes, including information about other parts of the system that were touched as part of the MR and need to be tested to ensure nothing is broken
• Scanned over my pull request in GitHub and addressed any warnings from the GitHub Build and Test checks in the GitHub PR ‘Files Changed’ tab
  Either:
• Documented my work in Confluence, updating any business rules applied or modified. Updated GitHub readme/documentation for the repository if appropriate. List of documentation links added/changed:
  • doc_1_here
    Or:
• Confirmed that none of the work that I have undertaken requires any updates to documentation

[kevwhitt-hee]

There are a few changes included in this PR that shouldn't be there, I believe. Please could this be addressed?

[kevwhitt-hee]

This file shouldn't be deleted.

[kevwhitt-hee]

@frank-hee give me a nudge when the above has been addressed, and I'll pull your branch and test.

[kevwhitt-hee]

Looks good. Built branch, worked through application screens in mobile and desktop and saw no styling issues.