Previously, checkboxes were only being marked _indeterminate_
if the default ("placeholder") value was true.  They should be
indeterminate whenever this is no explicitly set value.

Also lower the opacity for indeterminate checkboxes, to make them more
visually distinctive.

- Test devserver when serving hidden pages and their children
- Test URL resolution below hidden pages
- Add tests that children of hidden pages get built
- Test that artifacts of hidden pages get pruned

Even though hidden records produce no artifacts, they may have
non-hidden children.

See lektor#203

There may be non-hidden children below hidden pages.

One can now type the <delete> or <backspace> key to "unset" the
checkbox.

This is useful for the `_hidden` field where, _unset_ has different
semantics than `false`.

Previous method of checking the `checked` HTML attribute doesn't work
since React Controlled Components never update the `checked` HTML
attribute after the initial mount.