message: Show only preview buttons in directory for guest users
  This aligns the room directory with the rest of Element Web so that it does not
  allow guest users to join rooms. Instead, we show preview buttons (even when the
  preview might not actually happen).
  Fixes
diff: @@ -487,7 +487,11 @@ export default class RoomDirectory extends React.Component {
  let previewButton;
  let joinOrViewButton;
  - if (room.world_readable && !hasJoinedRoom) {
  + // Element Web currently does not allow guests to join rooms, so we
  + // instead show them preview buttons for all rooms. If the room is not
  + // world readable, a modal will appear asking you to register first. If
  + // it is readable, the preview appears as normal.
  + if (!hasJoinedRoom && (room.world_readable || isGuest)) {
  previewButton = (
  <AccessibleButton kind="secondary" onClick={(ev) => this.onPreviewClick(ev, room)}>{_t("Preview")}</AccessibleButton>
  );
  @@ -496,7 +500,7 @@ export default class RoomDirectory extends React.Component {
  joinOrViewButton = (
  <AccessibleButton kind="secondary" onClick={(ev) => this.onViewClick(ev, room)}>{_t("View")}</AccessibleButton>
  );
  - } else if (!isGuest || room.guest_can_join) {
  + } else if (!isGuest) {
  joinOrViewButton = (
  <AccessibleButton kind="primary" onClick={(ev) => this.onJoinClick(ev, room)}>{_t("Join")}</AccessibleButton>
  );
  