Switch to using LazyCollection / Generators

[godismyjudge95]

This PR reworks how the entries/users are retrieved and added to the export to use the FromGenerator concern - https://docs.laravel-excel.com/3.1/exports/from-generator.html

Without using the Generator exporter all the entries must be loaded into memory (in a collection) which can lead to hitting the memory limit when exporting a large number of entries.

I've kept basically all the same logic it just moves things around so that it is compatible with generators/lazy collections.

Let me know if there is something else you'd like me to change.