msglist/dm: Provide proper feedback when sending DMs to a deactivated user

[sm-sayedi]

When trying to send a DM message to a deactivated user, the message goes into void and nothing happens so the user knows they're exchanging messages with a deactivated user.

Here's how it looks right now:

This is how the web handles it:

The react-native mobile app just shows an infinite loading indicator on the right side of the message.

[gnprice]

Thanks! Agreed, we should fix this.