Add support for fragment redirects

[ehuss]

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?

Closes #2256

[ehuss]

@GuillaumeGomez or @notriddle I don't suppose either of you would be willing to give this a light review?

One minor concern I had was about preserving the fragments on deleted pages, since that is somewhat of an open redirect. So for example, old-page.html#foo will now redirect to new-page.html#foo and preserve the #foo part in the redirect. Would you agree that should be safe?

[GuillaumeGomez]

Is there a reason in particular we want to support this feature in the first place? Since mdbook is a static documentation generator, I'm not sure mdbook should be the one handling this case at all and should let it up to the server doing the rendering if any.

[ehuss]

We don't have a reasonable ability to define server redirects with the rust-lang documentation.

[GuillaumeGomez]

No we don't, and I'm not sure if we should. We keep older versions available (although my proposals on providing a GUI way to switch between versions were rejected) so that should cover this problem imo.