Exclude deactivated users in new-DM screen

[gnprice]

In the "New DM" screen, if you type a query that matches some deactivated users, they'll appear in the list.

If you then select them and go to the message list, the actual compose box is absent, replaced with an error banner — because you can't actually send a DM to a deactivated user. So we should leave them out of the "New DM" screen's list of users in the first place.