Now that the "All Crates" dropdown is only rendered on the search results page,
there is no need to load crates.js on most pages. Load it only on crate pages.
Also, add the `defer` attribute so it does not block page rendering.
For sidebar-items.js, move the script tag to `<head>`. Since it already has the
defer attribute it won't block loading. The defer attribute does preserve
ordering between scripts, so instead of the callback on load, it can set a
global variable on load, which is slightly simpler. Also, since it is required
to finish rendering the page, beginning its load earlier is better.
Remove generation and handling of sidebar-vars. Everything there can be computed
with information available in JS via other means.
Remove the "other" wrapper in the sidebar. It was unnecessary.
Remove excess script fields
