load crate menu contents on demand

[jsha]

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 #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:

docs.rs/src/web/crate_details.rs

Lines 90 to 129 in 19b354a

	let query = "
	SELECT
	crates.id AS crate_id,
	releases.id AS release_id,
	crates.name,
	releases.version,
	releases.description,
	releases.dependencies,
	releases.readme,
	releases.description_long,
	releases.release_time,
	releases.build_status,
	releases.rustdoc_status,
	releases.archive_storage,
	releases.repository_url,
	releases.homepage_url,
	releases.keywords,
	releases.have_examples,
	releases.target_name,
	repositories.host as repo_host,
	repositories.stars as repo_stars,
	repositories.forks as repo_forks,
	repositories.issues as repo_issues,
	repositories.name as repo_name,
	releases.is_library,
	releases.yanked,
	releases.doc_targets,
	releases.license,
	releases.documentation_url,
	releases.default_target,
	releases.doc_rustc_version,
	doc_coverage.total_items,
	doc_coverage.documented_items,
	doc_coverage.total_items_needing_examples,
	doc_coverage.items_with_examples
	FROM releases
	INNER JOIN crates ON releases.crate_id = crates.id
	LEFT JOIN doc_coverage ON doc_coverage.release_id = releases.id
	LEFT JOIN repositories ON releases.repository_id = repositories.id
	WHERE crates.name = $1 AND releases.version = $2;";

. 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:

docs.rs/src/web/crate_details.rs

Lines 207 to 213 in 19b354a

	let owners = conn.query(
	"SELECT login, avatar
	FROM owners
	INNER JOIN owner_rels ON owner_rels.oid = owners.id
	WHERE cid = $1",
	&[&crate_id],
	)?;

This query in releases_for_crate, across the releases table (with no LIMIT) could also be avoided at page load time:

docs.rs/src/web/crate_details.rs

Lines 247 to 260 in 19b354a

	let mut releases: Vec<Release> = conn
	.query(
	"SELECT
	id,
	version,
	build_status,
	yanked,
	is_library,
	rustdoc_status
	FROM releases
	WHERE
	releases.crate_id = $1",
	&[&crate_id],
	)?

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.

[syphar]

While this is definitely a valid optimization, we probably won't need it when the pages are fully cached. Even a full invalidation including old version for every release on each crate doesn't hurt much.