PLT-4530 Fix at mentions in edit post modal (#4449)

webapp/components/edit_post_modal.jsx

@@ -234,6 +234,7 @@ export default class EditPostModal extends React.Component {
                                 onKeyPress={this.handleEditKeyPress}
                                 onKeyDown={this.handleKeyDown}
                                 value={this.state.editText}
+                                channelId={this.state.channel_id}
                                 createMessage={Utils.localizeMessage('edit_post.editPost', 'Edit the post...')}
                                 supportsCommands={false}
                                 id='edit_textbox'