extract Admin::UsersController#index to its own query class

extract Admin::UsersController#index to its own query class
- move query to its own class
- use postgres ILIKE case insensitive
- removed duplicated list of trust levels

*This is a refactoring only:* not sure if this is a direction discourse wants to go? introduces more objects and more lines of code

Code Climate grade:
- new class AdminUserIndexQuery: A
- existing class Admin::UsersController: improves from B to A