Fix "private"/"PM" stragglers on the internal side of our API bindings

[gnprice]

This is a partial revival of:

• Say "DM" / "direct message" instead of "PM" / "private message", in code and UI #145

@PIG208 noticed at #145 (comment) that we have MessageType.private, which uses the old nomenclature. It should be MessageType.direct.

Similarly we have InitialSnapshot.recentPrivateConversations; it should be InitialSnapshot.recentDmConversations.

Both of those names reflect what's in the server API. We should use JsonKey and friends so that we continue to interact with the API in the same way, and just have the modernized names within our code.

After doing those two renames, rerun this search command to look for any remaining stragglers:

$ git grep -P 'pm|Pm|PM|(?i:private)' -- lib test