This updates everything for the move of utils to mdbook-core. There will
be followup commits that will be moving and refactoring these utils.
This simply moves them over unchanged (except visibility).
This adds the ability to redirect URLs with `#` fragments. This is
useful when section headers get renamed or moved to other pages.
This works both for deleted pages and existing pages.
The implementation requires the use of JavaScript in order to manipulate
the location. (Ideally this would be handled on the server side.)
This also makes it so that deleted page redirects preserve the fragment
ID. Previously if you had a deleted page redirect, and the user went to
something like `page.html#foo`, it would redirect to `bar.html` without
the fragment. I think preserving the fragment is probably a better
behavior. If the new page doesn't have the fragment ID, then no harm is
really done. This is technically an open redirect, but I don't think
that there is too much danger with preserving a fragment ID?
