Fixes wrongly generated relative urls for pagination.

When `RELATIVE_URLS` is `True` and the `PAGINATION_PATTERNS` adds folder to the path, the relative urls to these pages does not respect these additional folders.

Example:
	Settings:
			RELATIVE_URLS = True
			PAGINATION_PATTERNS = (
					(1, '{base_name}/', '{base_name}/index.html'),
					(2, '{base_name}/page/{number}/', '{base_name}/page/{number}/index.html'),)

	Theme:
			<link href="{{ SITEURL }}/theme/css/style.css" rel="stylesheet">

If you are on page 2 then "{{ SITEURL }}/theme/css/style.css" expands to "./theme/css/style.css" instead of "./../../theme/css/style.css".

Fix:
Simply compute the relative url from the paginated url instead of the not paginated one.
Merge pull request #1366 from Scheirle/paginated_relative_urls_fix2

Fixes wrongly generated relative urls for pagination.