Simplify references to VerificationRequest (#11045)

* Use new `VerificationRequest.getQRCodeBytes()`

* Use new `VerificationRequest.otherDeviceId`

* Remove references to `VerificationRequest.channel`

* Replace references to `VerificationRequest.{requesting,receiving}UserId`

Normally these are guarded by `request.initiatedByMe` so we can trivially
replace it with `request.otherUserId` or `client.getUserId()`. In one place we
actually need to apply some logic.

* increase test coverage

* Even more test coverage

* Even more test coverage

src/components/structures/MatrixChat.tsx

@@ -1667,7 +1667,7 @@ export default class MatrixChat extends React.PureComponent<IProps, IState> {
                 );
             } else if (request.pending) {
                 ToastStore.sharedInstance().addOrReplaceToast({
-                    key: "verifreq_" + request.channel.transactionId,
+                    key: "verifreq_" + request.transactionId,
                     title: _t("Verification requested"),
                     icon: "verification",
                     props: { request },