Add support/tests for Collection/Community role-based groups for Community/Collection Admins

This ticket originated in the discussion in https://github.com/DSpace/DSpace/pull/2722#discussion_r402937196, where we realized we need to have dedicated tests run as community/collection admins but also as normal admin/users for adding and removing of members from such group as well as edit the group metadata.

However, after that PR, we also realized that Community/Collection Admins currently cannot manage Community/Collection groups (they are only manageable by full Admins at this time). Therefore, we've extended this ticket to include adding the ability for Community/Collection Admins to manage Groups belonging to their Community/Collection. This also would require Community/Collection Admins to be able to view/search all other Groups / EPeople in order to add them as members of these Role-based groups.