Add an ID to the Search Channel Field (#6427)

* Add an ID to the Search Channel Field

For UI Automation

* Update searchable_channel_list.jsx

webapp/components/searchable_channel_list.jsx

@@ -178,6 +178,7 @@ export default class SearchableChannelList extends React.Component {
                 <div className='filter-row'>
                     <div className='col-sm-12'>
                         <input
+                            id='searchChannelsTextbox'
                             ref='filter'
                             className='form-control filter-textbox'
                             placeholder={localizeMessage('filtered_channels_list.search', 'Search channels')}