Circles, pub & auth

* Adds a boolean to each Circle, saying that
  it is public, and its existence may be known by any interested in
* Adds claims of type "YavscClaimTypes.CircleMembership" at
  login, in order to implement some faster authorisation processes and file restricted accesses.

src/Yavsc.Server/ViewModels/Auth/ClaimTypes.cs

@@ -4,6 +4,8 @@
 namespace Yavsc {
     public static class  YavscClaimTypes {
 
+        public const string CircleMembership = "CircleMembership";
+
         public const string GoogleUserId = "GoogleUserId";
     }