Switch labs to the handbook theme

[rufuspollock]

Propose switching labs to our increasingly standard "handbook" theme:

https://github.com/okfn/handbook-theme

[danfowler]

@rgrp The Labs website is certainly in need of a refresh which we are planning on at any rate. I'm not sure the "handbook" theme really applies here, though.

[rufuspollock]

@danfowler what are your concerns about using it?

The theme is quite nice and generic and there is nothing especially handbook-y about it (it is just called that because it was originally developed for the handbook).