Crown favicon optional

[davidread]

What should change

The crown favicon should not be used if configured to show_govuk_logo: false

Identifying a user need

I could be wrong, but the crown image is not a general use "government" logo, but the branding for GOV.UK, i.e. official information for citizens. And tech docs sites like https://tdt-documentation.london.cloudapps.digital/ and others we have in our department, are not on service.gov.uk domains, so don't come under the GOV.UK umbrella. The concern about using the logo on these sites would be the dilution of the GOV.UK brand and so trust in government information we publish.