Review back link and breadcrumb spacing and positioning

[anandamaryon1]

What

Currently the back link component requires the use of modifier classes to add the correct margins around it. The breadcrumbs component differs in that it contains its own padding and therefore must be placed in a different location within the HTML than the back link, which is inconsistent.

Therefore, it looks like we should consider updating the two components together, updating their spacing and defining their position within the HTML. This maybe require changes to the page template guidance too.

Related issues

Back link #32
Breadcrumbs: #6
Page template: #200
nhsuk-frontend PR: nhsuk/nhsuk-frontend#964

[anandamaryon1]

Released as part of v7.0.0: nhsuk/nhsuk-service-manual#2040