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Add test for riak_cs_acl_utils:add_grant/2

I've been suspecting that adding the same grant wasn't always
idempotent. This test seems to suggest it isn't.
Fix bug in riak_cs_acl_utils:add_grant/2

`lists:splitwith` only splits on the first occurrence. `lists:partition`
provides the expected behavior here.
Fail on invalid email address
Comment EQC test
Add test for `has_permission`
Add owner argument to `specific_acl_grant`
WIP
Grab display name for canonical id
Better `promote_failure` type spec
WIP
WIP
Add unexpected content error
Add helper funs for wm acl stuff
Fix spec for `has_canned_acl_header`
Don't allow ACL headers and body

Respond with HTTP 400 if the user provides both ACL headers and a body
to the ACL subresource for either an object or bucket.

TODO: should we cover this for multipart upload too? Or is that just
covered with the object resource already?