This replaces the `{{#previous}}` and `{{#next}}` handelbars helpers
with simple objects that contain the previous and next values. These
helpers have been a bit fussy to work with and have caused issues in the
past. This drops a large amount of somewhat fragile code with something
that is a bit simpler.
Additionally, this switches the previous/next arrows to use an `{{#if}}`
instead CSS trickery which may help with upcoming changes to
font-awesome.
This removes the deprecated `output.html.copy-fonts` option. This was
deprecated in https://github.com/rust-lang/mdBook/pull/1987. The
behavior now is that the default fonts are copied over unless there is a
custom `theme/fonts/fonts.css` file.
This is a pure git rename in order to make sure that git can follow
history. The next commit will integrate these into mdbook-html.
Additional commits will refactor/move/remove items.
