Added blocks to allow standard layout titles to be overridden

I was trying to integrate some pages that use Sonata with pages that don't; I want the Sonata heading bar on my non-Sonata pages too. This mostly worked but I also wanted to be able to set the page title without copying and pasting the whole template.

To allow this, I have added `{% block head_title %}` and `{% block page_title %}`.

(I didn't use 'title' because that's probably already used by people in their template hierarchy). Let me know if I should do this differently. :)