Add sign-up button to logged-out web UI (#19250)

app/javascript/mastodon/features/ui/components/sign_in_banner.js

@@ -1,10 +1,12 @@
 import React from 'react';
 import { FormattedMessage } from 'react-intl';
+import { registrationsOpen } from 'mastodon/initial_state';
 
 const SignInBanner = () => (
   <div className='sign-in-banner'>
     <p><FormattedMessage id='sign_in_banner.text' defaultMessage='Sign in to follow profiles or hashtags, favourite, share and reply to posts, or interact from your account on a different server.' /></p>
     <a href='/auth/sign_in' className='button button--block'><FormattedMessage id='sign_in_banner.sign_in' defaultMessage='Sign in' /></a>
+    <a href={registrationsOpen ? '/auth/sign_up' : 'https://joinmastodon.org/servers'} className='button button--block button-tertiary'><FormattedMessage id='sign_in_banner.create_account' defaultMessage='Create account' /></a>
   </div>
 );