Don't display the signin button until we've finished loading the page.

I noticed this whilst investigating the intermittent failures that we've been getting - now we've got additional delays loading the config and the persona file, we shouldn't really have the sign-in button available.

This may help the intermittents, though I did still see the same failure after adding this change, so I don't think it is a complete fix.

There's also a future change necessary to not show sign-in until after we've exhausted our log in possibilities, when we do this, we should also consider a better UI than just not showing the sign-in button.