Add www to brand.firefox.com, IO-3544 redux

[gcoxmoz]

A request for an uglifier www. as a prepended domain came in after the initial commit, sorry for the revisit.

Refractr PR Checklist

JIRA ticket: IO-3544 / #417

When creating a PR for Refractr, confirm you've done the following steps for a smooth CI and CD experience:

• Have you updated the relevant YAML in the PR?
• Have you checked the relevant YAML for any possible dupes regarding your domain?
• Have you checked if there are any TLS cert concerns - e.g. if the domain being redirected already exists, and it is being changed to point at refractr, is a temporary TLS 'outage' while waiting for certification via HTTP challenge okay? If not, add a note to the JIRA ticket.
• If desired, have you generated the nginx config manually to confirm updates work as expected?

After PR merge, next steps include:

• A merge to the main branch will automatically deploy refractr's stage environment -- deploying the prod environment requires a GitHub release to be created.
• Once deployed, refractr's certmap must be updated and DNS entries must be changed -- SRE can help with this. Please pull someone in on the JIRA ticket or ask for help in #sre on Slack.

[bkochendorfer]

Is there a link to the request? We avoid adding www subdomains on to an existing subdomain unless this is existing or previous art that we need to accommodate for.

[gcoxmoz]

Yes, in IO-3544. Even including -my- suggesting not to. :)

[bkochendorfer]

If that domain does not already exist let's not create it.

[gcoxmoz]

I accept this Uno Reverse.