fix(chat) hides send private chat button when enable.chat flag is false.

react/features/chat/components/PrivateMessageButton.js

 // @flow
 
+import { CHAT_ENABLED, getFeatureFlag } from '../../base/flags';
 import { translate } from '../../base/i18n';
 import { IconMessage, IconReply } from '../../base/icons';
 import { getParticipantById } from '../../base/participants';
  * @returns {Props}
  */
 export function _mapStateToProps(state: Object, ownProps: Props): $Shape<Props> {
+    const enabled = getFeatureFlag(state, CHAT_ENABLED, true);
+    const { visible = enabled } = ownProps;
+
     return {
-        _participant: getParticipantById(state, ownProps.participantID)
+        _participant: getParticipantById(state, ownProps.participantID),
+        visible
     };
 }