Register the /oil-and-gas prefix with the router

Register the browse pages at /oil-and-gas are registered with the
router. A prefix route is used so that subcategory pages are also
matched.

The registration of this route only takes place where the feature
flag 'industry_sectors_browse_enabled' is set to true.
Merge pull request #504 from alphagov/register-oil-and-gas-with-router

Register the /oil-and-gas prefix with the router