Consolidate private chats labelling

[dtonon]

We need to uniform the direct messages labeling; on the menu it is called "private chats" (I think it was a my proposal) while around the app there is still the old "Direct Messages" definition. I see that a lot of app just calling them "Messages", at the end I would use this, and maybe where we have space (e.g. the title of the section) use the expanded version "Private Messages" to suggest the encrypted nature (would be nice to have a contextual help with a summary about the e2e technology).