Add default accessible label to templates using localgov_base:prev-next as needed

[ctorgalson]

Currently localgov_base lacks these--IIRC, we removed one from one of the consuming templates when componentizing the Prev-Next nav, but didn't add it back. We can add them by default.

There are recent fixes to the component's prevNextAttributes variable that can enable us to add e.g. aria-label and aria-labelledby. Using aria-label is thus possible immediately, but following Drupal's usual <nav> convention is not currently possible.

[tonypaulbarker]

These changes were merged. Closing.