Refactor CreateCrossSigningDialog

* Converts CreateCrossSigningDialog to a functional component
 * Pulls logic out to its own class
 * Updates usage of deprecated cross signing bootstrap method on client to be on the crypto object and updates test to match

Moved from https://github.com/element-hq/matrix-react-sdk/pull/131

test/test-utils/test-utils.ts

@@ -129,6 +129,7 @@ export function createTestClient(): MatrixClient {
             getVerificationRequestsToDeviceInProgress: jest.fn().mockReturnValue([]),
             setDeviceIsolationMode: jest.fn(),
             prepareToEncrypt: jest.fn(),
+            bootstrapCrossSigning: jest.fn(),
         }),
 
         getPushActionsForEvent: jest.fn(),