In TopicController, in addition to ensure_can_move_posts!, we also
checked if the topic is private message in this line:
```ruby
raise Discourse::InvalidAccess if params[:archetype] == "private_message" && !guardian.is_staff?
```
However, this was not present in `guardian.can_move_posts?`. As a result,
the frontend topic view got an incorrect serialized result, thinking
that TL4 could move the private message post. In fact, once they tried
to move it, they got the `InvalidAccess` error message.
This commit fixes that TL4 will no longer sees the "move to" option in
the "select post" panel for a private message.
741e9d70ad