Add post-login complete security flow

This adds a step after login to complete security for your new session. At the
moment, the only verification method is entering your SSSS passphrase, but nicer
paths will be added soon.

This new step only appears when crypto is available and the account has
cross-signing enabled in SSSS.

Fixes https://github.com/vector-im/riot-web/issues/11214

res/css/_components.scss

@@ -28,6 +28,7 @@
 @import "./structures/_TopLeftMenuButton.scss";
 @import "./structures/_UploadBar.scss";
 @import "./structures/_ViewSource.scss";
+@import "./structures/auth/_CompleteSecurity.scss";
 @import "./structures/auth/_Login.scss";
 @import "./views/auth/_AuthBody.scss";
 @import "./views/auth/_AuthButtons.scss";