Twenty Fifteen: Missing border between post navigation items

[iamtakashi]

Since the_post_navigation() currently prints a next post first, the style made pre the_post_navigation hasn't been working. I'm not sure the fate of 29808.8.diff that flips the order and this is not necessary if the patch gets committed but, either way, if we use div rather than the class names we don't have to worry about it.

Related: #29808

Current:

[iamtakashi]

After:

[iandstewart]

In 30479:

Twenty Fifteen: bring back the border between post navigation links we lost in the_post_navigation changes.

Props iamtakashi, fixes #30414.