Improve error display for messages sent from insecure devices (#93)

* Add labs option to exclude unverified devices

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.

* DecryptionFailureBody: better error messages

Improve the error messages shown for messages from insecure devices.

* playwright: factor out `createSecondBotDevice` utility

* Playwright test for messages from insecure devices

* fixup! DecryptionFailureBody: better error messages

Use compound colour tokens, and add a background colour.

* fixup! DecryptionFailureBody: better error messages

Use compound spacing tokens

playwright/e2e/crypto/utils.ts

@@ -377,3 +377,14 @@ export async function awaitVerifier(
         return verificationRequest.verifier;
     });
 }
+
+/** Log in a second device for the given bot user */
+export async function createSecondBotDevice(page: Page, homeserver: HomeserverInstance, bob: Bot) {
+    const bobSecondDevice = new Bot(page, homeserver, {
+        bootstrapSecretStorage: false,
+        bootstrapCrossSigning: false,
+    });
+    bobSecondDevice.setCredentials(await homeserver.loginUser(bob.credentials.userId, bob.credentials.password));
+    await bobSecondDevice.prepareClient();
+    return bobSecondDevice;
+}