Updated form for subsite domains #3887

Nothing tricky so far.  Figuring out the UI was the hard part.

Three cases:
  - Site has a parent, user has change vsite domain access
  - Site has a parent
  - Site has no parent, but user has change viste domain access

It was the first case that slowed me down.  I started with a select full of possible domains.  One option was custom.  When custom was selected, the custom domain form we know and love would appear.  Getting #states right took a while, but even then it felt clunky.

Instead, any time a user has access to change the vsite domain, they get the textfield we all know.  Users get the dropdown if and only if their parent site offers domains and the user does not have domain access.

One other UI change was that instead of none or blank, I'm using the site's purl_base_domain to represent that there's no custom domain.  This seems clearer.

Now that that's squared away I just have to process the new domains.  Most of the same logic should apply, except for that purl_base_domain meaning no domain at all.