tabs: add clearURL method, loading tabs clears previous qps

Navigatable has clearURL, which for Search clears "search" qp, and
invites clears "invite" qp. Tab interfaces optionally include
"contentObject: AsTab", and show/hide funcs are passed the contentObject
of the previously loaded tab if one is available, so that they can call
it's clearURL method. This means searches you typed for the accounts tab
won't pop up when switching to activity.

ts/modules/accounts.ts

@@ -2516,6 +2516,10 @@ export class accountsList extends PaginatedList implements Navigatable, AsTab {
             if (details) this.details(details);
         });
     };
+
+    clearURL() {
+        this._search.clearURL();
+    }
 }
 
 // An alternate view showing accounts in sub-lists grouped by group/label.