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/Migrations/ApplicationDbContextModelSnapshot.cs

@@ -1107,6 +1107,8 @@ namespace Yavsc.Migrations
 
                     b.Property<string>("OwnerId");
 
+                    b.Property<bool>("Public");
+
                     b.HasKey("Id");
                 });