Replaced theme_more_link() with #type 'more_link'

Summary

• theme_link() was removed in #1985470: Remove theme_link() to stop dedicating theme functions to tiny, single tag chunks of markup because it's overkill and would only get worse if we were to create a Twig template.
• This extends the #type => link with a default title t('More') and class attribute .more-link.

Before:

array(
  '#theme' => 'more_link',
  '#url' => 'user',
  '#title' => 'Themed output generated in a KernelEvents::REQUEST listener',
);

After:

array(
  '#type' => 'more_link',
  '#href' => 'user',
  '#attributes' => array('title' => 'Themed output generated in a KernelEvents::REQUEST listener'),
);