Memebrships: Refactor Store interface

Use unified memberships table.
Add few internal API endpoints.

Change-Id: I80ac5a0f5c262e04d7898cca571b938a35d68d39
diff --git a/core/auth/proxy/unauthorized.html b/core/auth/proxy/unauthorized.html
index 130cf81..28c8913 100644
--- a/core/auth/proxy/unauthorized.html
+++ b/core/auth/proxy/unauthorized.html
@@ -15,7 +15,7 @@
             <p>
                 Only members of
                 {{ range $index, $group := .Groups }}
-                <a href="{{ $.MembershipPublicAddr }}/group/{{ $group }}">{{ $group }}</a>{{ if not (IsLast $index $.Groups) }},{{ end }}
+                <a href="{{ $.MembershipPublicAddr }}/group/{{ $group.Id }}">{{ $group.Title }}</a>{{ if not (IsLast $index $.Groups) }},{{ end }}
                 {{ end }}
                 are authorized to access this page.
             </p>