This changes all HTML IDs so that they have the `mdbook-` prefix. This
should help avoid ID conflicts between internal IDs and IDs from user
content such as section headers.
This is a relatively disruptive change and has a high risk of breaking
something. However, I think I have covered everything, and if anything
is missed, hopefully it will get detected.
I did not change class names since the chance of a collision is much
smaller than with IDs. However, that is something that could be
considered in the future.
Closes https://github.com/rust-lang/mdBook/issues/880
