urlpaths: seemingly full functionality

various subpath combos seem to work, and trailing slashes from them are
trimmed (including for the empty admin path "/", which is now "" by
default).

ts/user.ts

@@ -660,7 +660,7 @@ document.addEventListener("details-reload", () => {
             expiryCard.expiry = details.expiry;
 
             const adminBackButton = document.getElementById("admin-back-button") as HTMLAnchorElement;
-            adminBackButton.href = window.location.href.replace(window.pages.MyAccount, window.pages.Admin);
+            adminBackButton.href = window.pages.Base + window.pages.Admin;
 
             let messageCard = document.getElementById("card-message");
             if (details.accounts_admin) {