Use inline-block on page navigation links

Assumption: display:block was being used to enable the vertical padding
to apply. This occasionally causes layout errors (see zendesk 
ticket 543226). "Fix" this by making anchors inline-block.

Side effects: page navigation link hit zone no longer continues
ridiculously far to the right.
Merge pull request #370 from alphagov/probably-better-page-nav

Use inline-block on page navigation links