In the post list, when we navigate to the page with ">" button, we
navigate to older posts.
In the post view, when we navigate to the page with ">" button, we
navigate to older posts as well.
However, in the post list, the ">" button is called "next page".
At the same time, in the post view, the ">" button was called "previous
post". Now it's called "next post".
The difference isn't visible to normal users nor even API consumers as
the "get posts around post X" request isn't documented.
The change is motivated not only by consistency, but to also improve
compatibility with Vimperator's `[[` and `]]`. Vimperator assumes the
word "next" refers to ">" and the word "previous" refers to "<".
