Scope users listing and search to current company

Add scopeWhereCompany() to User model using whereHas through the
user_company pivot table. Apply it in UsersController::index() and
SearchController so users only see members of their current company.

Previously, the users page showed ALL users across all companies.

Ref #574

app/Http/Controllers/V1/Admin/General/SearchController.php

@@ -25,7 +25,8 @@ class SearchController extends Controller
             ->paginate(10);
 
         if ($user->isOwner()) {
-            $users = User::applyFilters($request->only(['search']))
+            $users = User::whereCompany()
+                ->applyFilters($request->only(['search']))
                 ->latest()
                 ->paginate(10);
         }