Add logged-out access to the web UI (#18961)

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

@@ -0,0 +1,11 @@
+import React from 'react';
+import { FormattedMessage } from 'react-intl';
+
+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>
+  </div>
+);
+
+export default SignInBanner;