Confine the search menu to be at most the width of the page, or 80% of
the viewport width, whichever is smaller. To do this, introduce a new
`--container-width` variable for the descendant elements of
`.ui.container` to be able to access.
Also update the relevant e2e test: add a long 'lorem ipsum' page, add a
search for it, parameterize the width.
