Right now we have buttons for "New conversation" and "New private message"
in different views, but both buttons do the same thing.
The current state is confusing for new users, since there is already a lot
of terminology one needs to learn in order to understand the Zulip
conversation model. It's very plausible a user would think a "conversation"
is something different from a "private message" or a "topic".
When a user clicks the compose `+` button, create a popover at the
bottom right of the screen including buttons for opening a new stream
message or a new private message.