Update existing full-page examples that have a service name or navigation to use the Service Header component instead.

Also update header and phase banner styles to work with service header:

- Makes header border full-width in all circumstances
- Adds content width container to phase banner

- Change a Notification Banner test that created a second banner role, as this created an accessibility failure with the header always being present in the template

Remove the 'with no options set' example from being tested as part of the standard accessibility tests.

This is because the accessibility test tries to initialise the component's JS, but there is no HTML being rendered to initialise it against, returning an ElementError that Jest considers to be a test failure.

@selfthinker noted that having the entire `<nav>` element toggled means that the navigation landmark is inaccessible when the menu is closed. Not having the toggle within the landmark also means that assistive technology users wouldn't be able to jump to it easily.

This commit moves the toggle button to within the `<nav>` element and tweaks the JS so that the `<ul>` has its visibility toggled instead.

The method by which component JS is loaded and initialised changed in Frontend 5.4.0. This commit updates some of the references that didn't get caught while rebasing.

Partially reverts the changes from 2c67ecc

These changes unfortunately broke some other use cases for the Service Header.

After some discussion, we’ve decided that the phase banner will remain in its existing position, rather than trying to include it within the new section, to avoid the complications it introduces.

The separation of the width container and service header container has been retained along with the moved start and end slots.

This reverts commit 8e67b18.

This fixes a few issues identified during testing, all caused by the
`inline-block` approach to inlining these items.

- Removes the extra whitespace from between each list item that
  screwed with the spacing between links and screen readers would
  pointlessly announce.
- Fixes an NVDA issue in Firefox and Chrome <= 124 where it would read
  all of the links as a run-on sentence.

Adds some margin-right to the service name on tablet breakpoint so that the service name and navigation links no longer touch in certain circumstances.

This could happen if the navigation and service name were collectively wide enough to fill the entire horizontal space at the present viewport size.

IE11 is tripping over the use of flex-wrap and is instead presenting all links as a single, horizontally scrolling row.

This commit reverts the use of flexbox on IE11 (and IE10) specifically, falling back to the previous inline-block behaviour. This means that the previous accessibility issues are now present in IE11 again, but having the navigation wrap seems like a higher priority usability issue to address.

This reverts commit c9b24e9.