DownloadsController and Ability mods

Factored out custom Ability behavior to DulHydra::Models::Ability.
Moved config of additions to ability_logic initialization to DulHydra.extra_ability_logic config setting.
Added basic datastream permissions to Ability `read_permissions` and `edit_permissions` methods.
Added `download_permissions` method to Ability.
Added spec test module for Ability model.
Added spec test module for User model.
Added spec test module DownloadsController.
Fixes #294.