Fix naming of DMs and Chat

DMs need to be known as Direct Messages and Chats need to be known as Chats. They are separate and should stay separate as much as possible.

• Rename DMs and Chats to avoid confusion. If they need to be split apart in the codebase we should do that as well. If that should be moved to a separate issue, we can.
• Remove Chats as an option for a reply scope to posts, because that's literally impossible to do anyway
• Chats need their own unique icon (see attached)

attached --->