Revert "Don't hard-code essential files in copy_doc_dir"

[jyn514]

This meant we stopped copying all JS and CSS files, which was
incorrect. Possibly there's a better way to do this, but this fixes the
immediate problem that search is broken.

Here's an example build which is broken because of the change to
copy_doc_dir:

> aws s3 ls s3://rust-docs-rs/rustdoc/tokio-eventfd/0.2.0/
                           PRE i686-unknown-linux-gnu/
                           PRE implementors/
                           PRE src/
                           PRE tokio_eventfd/
                           PRE x86_64-apple-darwin/
2021-03-23 14:25:56          9 .lock
2021-03-23 14:25:56       1691 settings.html

Re-opens #294.

r? @Nemo157

[jyn514]

I expect this to fix #1328, but haven't tested it yet.

[jyn514]

(The diff looks big, but it's a line-for-line revert of 6f4477b.)

[jyn514]

I expect this to fix #1328, but haven't tested it yet.

Oh weird, it does not. Looking into that now. This does fix search being broken though.