Super user invites (#483)

This PR brings user invites logic to the Management service
via HTTP API. 
The POST /users/ API endpoint creates a new user in the Idp
and then in the local storage. 
Once the invited user signs ups, the account invitation is redeemed.
There are a few limitations.
This works only with an enabled IdP manager.
Users that already have a registered account can't be invited.

management/server/grpcserver.go

@@ -181,7 +181,7 @@ func (s *GRPCServer) registerPeer(peerKey wgtypes.Key, req *proto.LoginRequest)
 			return nil, status.Errorf(codes.Internal, "invalid jwt token, err: %v", err)
 		}
 		claims := jwtclaims.ExtractClaimsWithToken(token, s.config.HttpConfig.AuthAudience)
-		_, err = s.accountManager.GetAccountWithAuthorizationClaims(claims)
+		_, err = s.accountManager.GetAccountFromToken(claims)
 		if err != nil {
 			return nil, status.Errorf(codes.Internal, "unable to fetch account with claims, err: %v", err)
 		}