Bump govuk_content_models to 6.0.3

This adds support for the new industry_sectors tag type in the
Artefact model.
Don't assume all tags have web_urls

Only tags in the 'section' tag type actually have public-facing webpages
on GOV.UK. Other tag types, such as 'legacy_sources' don't have these,
yet the Content API returns a similar looking URL pointing to a browse
page which does not exist.

This changes the URLHelper's with_tag_web_url method to check the tag
type before returning a URL, and return nil if it isn't a section tag.

Returning nil feels like the right behaviour because there isn't a
public web_url which exists for this entity. This change is unlikely to
break clients as there are currently very few tags which would be
affected by this change.
Return a custom web_url for industry_sector tags

When a tag is from the "industry_sector" type, return a web_url at the
root of GOV.UK.

The example of this would be the prototype Oil & Gas specialist topic
page, which currently will live at /oil-and-gas.
Merge pull request #145 from alphagov/change-tag-web-urls

Change how the web_url is built for a tag