First pass at an Oil & Gas sector page
Rename 'sectors' to 'industry_sectors'
Add an integration test for industry sectors

This currently relies on the 'content-api-moar-test-helpers' branch in
gds-api-adapters.
Add tests and Slimmer headers for industry sectors
Tweak the design and layout of industry sectors

This makes them look a bit more like the business browse page.
Add report-a-problem to the sector pages
Bump gds-api-adapters to 8.2.3

This includes fixes for the Content API test helpers.
Gracefully handle when a sector tag does not exist

When a request for an industry sector tag returns a 404 from the Content 
API, return a 404 status so that we render an error page.

I've extracted the 'api_returns_404_for' method from the browse 
functional tests into a file in the test/support/ directory so that 
other tests are able to use it.
Add full-width body class to sector pages
Include the parent sector in the title tag

This also changes the test data so that "Oil and gas" is consistently in 
sentence case.
Put the oil and gas route behind a feature flag
Tweaks to the sector and subcategory view templates

- Add a beta label to the sections
- Only show the category-description div if the description isn't empty
- Add the "specialist topic" label to the subcategory page
Merge remote-tracking branch 'origin/master' into oil-and-gas-sector

Conflicts:
	Gemfile
	Gemfile.lock
Check that sector browse slugs are valid

Before we pass sector slugs into a request to the Content API, make 
sure they look like valid slugs and don't contain obvious invalid 
characters.
Set expiry headers for sector browse pages
Merge pull request #501 from alphagov/oil-and-gas-sector

Add specialist browse pages for the Oil & Gas sector