Removes two 'narrow_to_recipient' banners which were
shown after sending a message to a different conversation.
Now, the sender is narrowed to the conversation where
message was sent.
Fixes#29186.
This also introduces a combo grid/flex layout, which will also be
applied to the DM and Streams headings.
Because there are now multiple classes referenced from the
Puppeteer tests, those selectors now include the expanded
navigation area's parent ID selector.
