Move .nhsuk overrides to new scss file and update layout

[frank-hee]

JIRA link

TD-5838

Description

all .scss file rules involving .nhsuk elements moved into one new file and reorganised for ease of future updates.

• LearningHub.Nhs.WebUI/Styles/nhsuk/common.scss - .nhsuk ovverrides removed
• LearningHub.Nhs.WebUI/Styles/nhsuk/layout.scss - .nhsuk ovverrides removed
• LearningHub.Nhs.WebUI/Styles/nhsuk/nhsuk-overrides.scss - created and .nhsuk overrides added
• LearningHub.Nhs.WebUI/Styles/nhsuk/nhsuk.scss - overrides moved from previous job TD-4916 as they were potentially causing a memory issue
• LearningHub.Nhs.WebUI/Views/Shared/Tenant/LearningHub/_Layout.cshtml - added link to nhsuk-overrides.css

Screenshots

Introduces no new errors to Axe and Wave tests

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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