Rollup merge of #48631 - focusaurus:remember-collapse-setting, r=QuietMisdreavus

Remember state of top-level collapse toggle widget

This change allows the big top-right expand/collapse toggle to remember its setting across navigation or page reloads. Prior to this change, there was this annoyance:

- browse to some docs
- Click the minus button to collapse them
- browse to other docs (or reload the page)
- Everything is expanded again

The solution is based on storing a simple boolean flag in localStorage. I think it's a good improvement, but it does introduce the following potentially surprising behavior:

- browse to some docs
- click the minus button to collapse them
- click to expand a particular item (not the main top-right big one)
- reload the page, everything is collapsed

Paired with @debugsteven on this.

src/librustdoc/html/static/main.js

@@ -1628,6 +1628,7 @@
     function toggleAllDocs() {
         var toggle = document.getElementById("toggle-all-docs");
         if (hasClass(toggle, "will-expand")) {
+            updateLocalStorage("rustdoc-collapse", "false");
             removeClass(toggle, "will-expand");
             onEveryMatchingChild(toggle, "inner", function(e) {
                 e.innerHTML = labelForToggleButton(false);
@@ -1637,6 +1638,7 @@
                 collapseDocs(e, "show");
             });
         } else {
+            updateLocalStorage("rustdoc-collapse", "true");
             addClass(toggle, "will-expand");
             onEveryMatchingChild(toggle, "inner", function(e) {
                 e.innerHTML = labelForToggleButton(true);
@@ -1988,6 +1990,10 @@
     window.onresize = function() {
         hideSidebar();
     };
+
+    if (getCurrentValue("rustdoc-collapse") === "true") {
+        toggleAllDocs();
+    }
 }());
 
 // Sets the focus on the search bar at the top of the page