Add `getShowSasCallbacks`, `getShowQrCodeCallbacks` to VerifierBase (#3422)
* Add `getShowSasCallbacks`, `getShowQrCodeCallbacks` to VerifierBase
... to avoid some type-casting
* Integration test for QR code verification
Followup to https://github.com/matrix-org/matrix-js-sdk/pull/3436: another
integration test, this time using the QR code flow
* Rename method
... it turns out not to be used quite as I thought.
* tests for new methods
* Use Object.defineProperty, and restore afterwards
Apparently global.crypto exists in some environments
* apply ts-ignore
* More test coverage
* fix bad merge