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/Users/UsersController.php

@@ -25,14 +25,15 @@ class UsersController extends Controller
 
         $user = $request->user();
 
-        $users = User::applyFilters($request->all())
+        $users = User::whereCompany()
+            ->applyFilters($request->all())
             ->where('id', '<>', $user->id)
             ->latest()
             ->paginate($limit);
 
         return UserResource::collection($users)
             ->additional(['meta' => [
-                'user_total_count' => User::count(),
+                'user_total_count' => User::whereCompany()->count(),
             ]]);
     }