The dark-mode-friendly SVG mask for the wizard's background image
introduced in 8fcfb9586c does not work with
CDNs, because CORS restrictions apply to SVG masks.
It would be complicated to modify CDN access origin rules for this one
specific assets, so instead, this PR moves the contents of the SVG file
inside the stylesheet.
Adds a "Step 0" to the wizard if the site has no admin accounts where
the user is prompted to finish setting up their admin account from the
list of acceptable email addresses.
Once confirmed, the wizard begins.
