Improve pagination include

[bewuethr]

In #176, the paginator include has been modified to not rely on any legacy paginate-v1 settings, enabling pagination of multiple collections.

The trail is still rather ugly, muddling together permalinks and replacing numbers; a nicer solution would be to leverage the paginator.page_trail. An example of an implementation can be found in mmistakes/minimal-mistakes#2636; notice the issues with the redundant index.html being appended.

While we're at it, whitespace could also do with a cleanup.