[api] create one unified relationship model

move the relationships of [user|group] and [package|project] into
one table
[api] optimize loading maintainers in /status/project
[api] retire {package|project}_group_role_relationship

prj.project_group_role_relationships is now prj.relationships.groups
(unless you look for a specific group then you it's just relationships)
[api] retire package_user_role_relationship model

self.relationships.users will do for packages, self.relationships.packages will do
for users
[api] retire project_user_role_relationships

self.relationships.users will do for projects, self.relationships.projects
will do for users

also remove the GroupRelationship model I added at some point
[api] avoid too big calls into memcached

Rails.cache.read_multi hangs "forever" with 13000 packages, so divide this
[api] move XML renderer to views where easily possible