navbar: use flex-none to avoid shrinking

On GNOME Web / Epiphany, the navbar starts shrinkig as the page grows in
length, leading to a difficult-to-use navbar. Let's avoid this by
explicitly requesting no flex behavior for this element.

It's unclear to me if Ephiphany or the other browsers are right here,
but this doesn't seem to break anything as far as I can tell.
5 jobs for master in 15 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #3011079
pages

00:00:15

 
  Deploy
passed pages:deploy

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failed pages:deploy

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failed pages:deploy

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failed pages:deploy

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