https://github.com/hypothesis/client/pull/2818 inadvertently changed the size
of the `.menu-toggle__icon` SVG element from 10x10px to 16x16px due to an
`svg` selector introduced by the `buttons.button--icon-only` mixin
taking precedence over the `.menu-toggle__icon` selector.

As a result, the caret's vertical position was incorrect and it did not
remain vertically centered when flipped.

This commit nests the `.menu-toggle__icon` selector inside the parent
`.menu-toggle` selector to restore the caret icon back to its 10x10
pixel size.