Masthead: Use inverse components styles from govuk-frontend v5.0.0 and adjust spacing

[paulrobertlloyd]

govuk-frontend v4.7.0 introduced inverse modifiers for buttons, breadcrumbs and back links, specifically due to use cases like that used by the Masthead component.

This means we can remove our own modifiers. Of note:

1. We still need additional adaptions for the breadcrumb and phase banner when they sit within this component (see Consider adding a bottom-border modifier for breadcrumbs alphagov/govuk-frontend#3914)

2. The govuk-frontend inverse button component specifies govuk-colour("blue") for the button text, not govuk-brand-colour.

   That might be the right decision so opening it up to discussion as to whether to stick to that within the previous behaviour (which requires overrides), or changing to use blue text. The MoD design system, as one example, also uses the brand colour for button text within the masthead).

   Update 27 July 2023: This issue is being addressed in Improvements to inverse components alphagov/govuk-frontend#3925

[paulrobertlloyd]

Have asked about intended inverse button text colour on X-GOV Slack.

[paulrobertlloyd]

Component revised such that:

• Use inverse styles now provided by govuk-frontend
• Use larger fonts by default and remove --large modifier
• Apply margins in the same direction
• Do less modification of child elements

These changes mean the design more closely aligns with both masthead on product pages, and also the topic pages used on GOV.UK.