Decouple redirects from language (#24597)

* experimenting with redirects

* cleanup developer.json

* wip

* clean console.log

* progress

* some progress

* progress

* much progress

* debugging tests

* hacky progress

* ditch latest -> number redirects

* minor

* hacky progress

* lots of progress

* some small fixes

* fix rendering tests

* small fixes

* progress

* undo debugging

* better

* routing tests OK

* more cleaning

* unit tests

* undoing lineending edit

* undoing temporary debugging

* don't ever set this.redirects on Page

* cope with archived version redirects

* adding code comments on the major if statements

* address all feedback

* update README about redirects

* delete invalid test

* fix feedback

lib/path-utils.js

@@ -33,7 +33,7 @@ export function getPathWithoutVersion(href) {
 }
 
 // Return the version segment in a path
-export function getVersionStringFromPath(href) {
+export function getVersionStringFromPath(href, supportedOnly = false) {
   href = getPathWithoutLanguage(href)
 
   // Return immediately if this is a link to the homepage
@@ -65,6 +65,16 @@ export function getVersionStringFromPath(href) {
     return allVersions[planObject.latestVersion].version
   }
 
+  // If the caller of this function explicitly wants to know if the
+  // version part is *not* supported, they get back `undefined`.
+  // But this function is used in many other places where it potentially
+  // doesn't care if the version is supported.
+  // For example, in lib/redirects/permalinks.js it needs to know if the
+  // URL didn't contain a valid version.
+  if (supportedOnly) {
+    return
+  }
+
   // Otherwise, return the first segment as-is, which may not be a real supported version,
   // but additional checks are done on this segment in getVersionedPathWithoutLanguage
   return versionFromPath