Twenty Fifteen: Use core navigation template tags

[obenland]

r30065 introduced navigational template tags for all themes to use. They are a bit different than what we use in Twenty Fifteen, but let's use them and serve as a best practice on that regard as well.

[iamtakashi]

By looking at what you said in Slack, it might have been intentional that you left image.php untouched but

Fatal error: Call to undefined function twentyfifteen_post_nav() in .../image.php on line 73

[iamtakashi]

Avoid a fatal error on image.php

[obenland]

Just noticed the patches use two different code styles regarding PHP tags between the_pagination() and the_post_navigation(). I think the former is preferred?

[iamtakashi]

With consistent PHP Tag style

[iandstewart]

In 30216:

Twenty Fifteen: Use new core navigation tags.

Props obenland, iamtakashi, fixes #30189