[feature] Add exclusion config for loaded resources

[elvinaspredkelis]

Currently Oversee loads all of the available ActiveRecord models. However, it's very unlikely that all of the models should be displayed in the admin dashboard (e.g. join tables are usually irrelevant for the admins).

Therefore, adding an exclusion config would be an amazing UX improvement. It would be ideal to come up with a dynamic way of setting such list (e.g. to load different resources for different moderation roles). However, it might be a tough thing to do for now, so an initializer is a viable solution for now.