load crate menu contents on demand

Most page navigations don't result in opening the crate menu. We could load that menu using JS, either immediately after page load or on click / hover.

That would simplify https://github.com/rust-lang/docs.rs/issues/1560 because a page's HTML wouldn't depend on the list of crate versions and yanked versions.

It would also, I believe, result in an improvement in performance, reducing database load and wall clock time to load rustdoc pages.

Right now, every page load invokes this JOIN query across releases, crates, doc_coverage, and repositories: https://github.com/rust-lang/docs.rs/blob/19b354ab0b910014c7bc11da811a40b19502cc87/src/web/crate_details.rs#L90-L129. If the initial page load doesn't need doc_coverage and repository info, we can reduce this to a JOIN across releases and crates.

Each page load also does a query across owners and owner_rels, which doesn't need to be done at page load time: https://github.com/rust-lang/docs.rs/blob/19b354ab0b910014c7bc11da811a40b19502cc87/src/web/crate_details.rs#L207-L213

This query in releases_for_crate, across the releases table (with no LIMIT) could also be avoided at page load time: https://github.com/rust-lang/docs.rs/blob/19b354ab0b910014c7bc11da811a40b19502cc87/src/web/crate_details.rs#L247-L260

So I think the net result would be replacing three queries (two of them using JOINs) with a single, simple query with a single result.

Start in src/web/crate_details.rs at the linked query sections and trace how crate menu contents are rendered. Compare the current page-load queries with the proposed on-demand loading and consider the relationship to issue 1560. Done means the initial page no longer requires the unnecessary menu data while the crate menu still loads with its existing contents.