Use compatibility flag for focus state colours and remove manual fixes

[maxgds]

https://trello.com/c/nhfpb4wh/139-revisit-government-frontend-in-light-of-knowing-about-the-govuk-compatibility-govuktemplate-flag

This switches to using the $govuk-compatibility-govuktemplate and removes other code that was implemented to get the correct focus states.

The only visual change should be an update to the focus state of buttons when they are both hovered/active and focused.

Button example page: https://www.gov.uk/settled-status-eu-citizens-families/applying-for-settled-status

Before

After

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Examples of pages affected by updates, where no visual change is expected:

• Back to top link: https://www.gov.uk/guidance/pregnancy-advice-on-contact-with-animals-that-are-giving-birth
• Download link: https://www.gov.uk/foreign-travel-advice/afghanistan (PDF link beneath image)
• Published dates: https://www.gov.uk/government/consultations/environmental-permitting-standard-rules-consultation-no-21
• Publisher metadata: https://www.gov.uk/government/consultations/environmental-permitting-standard-rules-consultation-no-21 (Environment Agency link)
• Part navigation: https://www-origin.integration.publishing.service.gov.uk/change-address-driving-licence (contents list below main header)
• White links: https://www.gov.uk/government/consultations/environmental-permitting-standard-rules-consultation-no-21 ("another website" link)