Switch off tabular numbers in headings

Using tabular numbers means that Chrome calculates the text width
wrongly, resulting in a weird display of our heading links.

We don't really need tabular numbers in headings, because we shouldn't
really have any numbers in headings. So let's switch them off.

This is a workaround for https://code.google.com/p/chromium/issues/detail?id=344347
Merge pull request #210 from alphagov/65713466-chrome-windows-title-rendering-issue

Switch off tabular numbers in headings