Minor improvements to poll composing UI (#12319)

- Disable the “add option” button instead of hiding it
- Allow poll option inputs to scale to full width

app/javascript/mastodon/features/compose/components/poll_form.js

@@ -142,9 +142,7 @@ class PollForm extends ImmutablePureComponent {
         </ul>
 
         <div className='poll__footer'>
-          {options.size < 4 && (
-            <button className='button button-secondary' onClick={this.handleAddOption}><Icon id='plus' /> <FormattedMessage {...messages.add_option} /></button>
-          )}
+          <button disabled={options.size >= 4} className='button button-secondary' onClick={this.handleAddOption}><Icon id='plus' /> <FormattedMessage {...messages.add_option} /></button>
 
           <select value={expiresIn} onChange={this.handleSelectDuration}>
             <option value={300}>{intl.formatMessage(messages.minutes, { number: 5 })}</option>