Add specialist browse pages for the Oil & Gas sector

This pull request adds a new kind of browse page for specialist users in an industry sectors. They are driven by the 'industry_sector' tag in the Content API, and contain any type of artefact (the majority of which will be detailed guides). Right now, we only display the browse page for the 'oil-and-gas' tag, and its child tags.

In terms of the requests made to the Content API, and the URL structure, industry sectors browse pages are quite similar to a regular section browse page. However, there isn't a `/browse` URL prefix, nor is there (currently) an index of all industry sectors.

This is intended to be released with a 'beta' label, mainly because the taxonomy is still being experimented with. The new browse pages are also behind a feature flag, which will default to 'off' in non-development and test environments. Once this has been merged, I'll open a pull request on alphagov-deployment to enable this in preview.