Treat users and groups as distinct concepts

To make it easier to reason about and document the new security behavior, this PR splits roles into users and groups.

* Users (but not groups) can be given authentication information (sources).
* Groups (but not users) can be used to cascade privileges to other groups and users.

Where the code does not or cannot distinguish between users and groups, I continue to refer to roles, but the user interface is constrained to users and groups.

There are 3 projects impacted by this change, all with the same branch name (`jrd-security-role-bifurcation`):
* riak
* riak_core
* riak_test

All should be merged at the same time. I will be filing PRs for each.

/cc @Vagabond

(riak_test note: currently `http_security` test does not reference groups, so no fixes are required.)