Add SSO providers list to UI at most once

Each of the current SSO extensions work by leveraging HTML patches to add an SSO providers list to the login UI, which each subsequent SSO extension adds to. Since this results in multiple copies of the list, CSS is used to hide all but the last of these, as only the last such list will have all expected entries.

This works, but the CSS involved relies on :last-child and will not behave as expected if anything else is added to the same area of the login UI. It would be far better if the SSO providers list were always added exactly once, regardless of how many SSO extensions are installed.