Add labs option to exclude unverified devices (#92)

Add a labs option which will, when set, switch into the "invisible crypto"
mode of refusing to send keys to, or decrypt messages from, devices that have
not been signed by their owner.

test/test-utils/test-utils.ts

@@ -131,6 +131,7 @@ export function createTestClient(): MatrixClient {
             resetKeyBackup: jest.fn(),
             isEncryptionEnabledInRoom: jest.fn(),
             getVerificationRequestsToDeviceInProgress: jest.fn().mockReturnValue([]),
+            setDeviceIsolationMode: jest.fn(),
         }),
 
         getPushActionsForEvent: jest.fn(),