Update to support current version of rustdoc.

https://github.com/rust-lang/rust/commit/acbfb8c3bdbcb29b23b11221ae2ec1386b061aa7

Fix busted check for undefined variable.

Fix failure to sync theme after https://github.com/rust-lang/rust/commit/727a1fd1945685f21d4d2b1d86dbf6429372f3ef#diff-8c049b94fec9a823ef7bd2e7d8d5ac32c7591be2804d7d754bf54b4e92e53c66

* A way to redirect standard library URLs in the docs to point at a local copy.

* Update options.html to match the latest tweaked version of the settings panel.

* Update to handle new version of rustdoc that uses document.write for themes.

Fix error on new, nightly version of rustdoc. Caused by https://github.com/rust-lang/rust/pull/96390/files#diff-0c4c5f4709d4f85219afe9ed48a00370c8de11eed5fb3ebee6c683ebb391a72bR173

Fix nightly rustdoc bug in updateLocalStorage

https://github.com/rust-lang/rust/commit/7efba33582ab226e64a8cbee73041d44e7789528#diff-0c4c5f4709d4f85219afe9ed48a00370c8de11eed5fb3ebee6c683ebb391a72bR109

Fix ordering bug that affects setups where system themes are turned off.

# Avoid subscribing to rustdoc sync until after startup

This is because rustdoc sets its localstorage at startup, and we need
to ignore all that or we get a race condition between addons settings
being loaded and rustdoc being loaded.