Use brand `logo` to set favicon in dashboards

[cwickham]

Seems brand: logo: small would be a perfect candidate for favicon.

Could still be overwritten by website: favicon

[cscheid]

This would be great to have in 1.6 and we might even backport it if we do it in 1.7, but I don't want any more feature PRs into 1.6, so it's going to 1.7

[mcanouil]

For the dashboard part:

• dashboard: favicon support #9126

[mcanouil]

Website/book now uses logo:

• [feature] brand - forward logo.small to favicon #12135

Since dashboard does not support favicon at all, brand logo is not yet supported.