Clean up usage of `ListView` controller

* Remove all non-mapping `list_loader` implementations except
  `model_list_loader` for Admin page
* Fix Admin initialization by using new `__page_only` query parameter to
  get counts without loading resources
* Use query parameters instead of multiple searches with a client-side
  merge
* Make usage of `paging` data uniform (not requiring `find_function`,
  but still using `list_view_options.find_params`)
* Reduce default paging size to 50 (from 100)
* Add attribute specification for `Event.modified_by`