Closes#4803
After this commit use `$titleblock$` in order to get what was contained
in `$title$` before, that is a title and subtitle rendered according to
the official rST method:
http://docutils.sourceforge.net/docs/user/rst/quickstart.html#document-title-subtitle. from
With this commit, the `$title$` and `$subtitle$` metadata are available and they
simply carry the metadata values. This opens up more possibilities in templates.
