FIX: raised a proper NotFound exception when filtering groups by username with invalid username.
FIX: properly filter the groups based on current user visibility when viewing another user's groups.
DEV: Guardian.can_see_group?(group) is now using Guardian.can_see_groups(groups) instead of duplicating the same code.
FIX: spec for groups_controller#index when group directory is disabled for logged in user.
FIX: groups_controller.sortable specs to actually test all sorting combinations.
DEV: s/response_body/body/g for slightly shorter spec code.
FIX: rewrote the "view another user's groups" specs to test all group_visibility and members_group_visibility combinations.
DEV: Various refactoring for cleaner and more consistent code.
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