Give SINGLE_USER a chance to register (#1820)

An attempt to open a brand new Mastodon instance configured
as SINGLE_USER_MODE=true will cause an exception.

Enable temporary registration if we have no users in the database

Fixes #1817

app/views/accounts/show.html.haml

@@ -14,7 +14,7 @@
   %meta{ property: 'og:image:height', content: '120' }/
   %meta{ property: 'twitter:card', content: 'summary' }/
 
-- if !user_signed_in? && !Rails.configuration.x.single_user_mode
+- if !user_signed_in? && !single_user_mode?
   = render partial: 'shared/landing_strip', locals: { account: @account }
 
 .h-feed