Rewrite <NotSignedInIndicator /> as FC (#24903)

app/javascript/mastodon/components/not_signed_in_indicator.jsx → app/javascript/mastodon/components/not_signed_in_indicator.tsx

 import React from 'react';
 import { FormattedMessage } from 'react-intl';
 
-const NotSignedInIndicator = () => (
+export const NotSignedInIndicator: React.FC = () => (
   <div className='scrollable scrollable--flex'>
     <div className='empty-column-indicator'>
       <FormattedMessage id='not_signed_in_indicator.not_signed_in' defaultMessage='You need to sign in to access this resource.' />