Make secondary www CNAME DNS check optional

[jvatic]

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When adding a domain to GitHub pages and verifying DNS, this can pass for the primary check, but show a warning for the secondary www CNAME check which can cause some confusion at first glance since docs.github.com doesn't mention this as a step that I might want or need to take, and becuase the www in the warning message title can be easy to miss since it's right next to the domain name I'm expecting to see (e.g. www.mydomain.top).

There's a few things I'd like to suggest:

1. Render www so that it pops out from the rest of the domain name in the displayed warning
2. Link to documentation on why the warning is showing up (someone from GH support mentioned this can happen when the domain had a www CNAME when it was first configured with GH Pages)
3. Allow skipping this check so that the warning doesn't show again, e.g.:
   1. A default-enabled checkbox that can be unchecked
   2. An action button in the warning message