accounts: infinite scroll for performance

Found out the bottleneck when ~2000 or more elements are loaded isn't
the search or sort or anything, but the DOM. An infinite scroll
implementation is added, where elements are added to the DOM as you
scroll. May still be a little buggy, and can't yet cope with screen
resizes. Also, the "shown" indicator is broken.

ts/admin.ts

@@ -192,7 +192,7 @@ login.onLogin = () => {
     window.updater = new Updater();
     // FIXME: Decide whether to autoload activity or not
     reloadProfileNames();
-    setInterval(() => { window.invites.reload(); accounts.reload(); }, 30*1000);
+    setInterval(() => { window.invites.reload(); accounts.reloadIfNotInScroll(); }, 30*1000);
     // Triggers pre and post funcs, even though we're already on that page
     window.tabs.switch(window.tabs.current);
 }