Add cross-signing diagnostic panel

This is not part of any designs, so it may be short-lived, but it's quite handy
for diagnosing issues with cross-signing at least while the feature is in
development.

res/css/_components.scss

@@ -171,6 +171,7 @@
 @import "./views/rooms/_Stickers.scss";
 @import "./views/rooms/_TopUnreadMessagesBar.scss";
 @import "./views/rooms/_WhoIsTypingTile.scss";
+@import "./views/settings/_CrossSigningPanel.scss";
 @import "./views/settings/_DevicesPanel.scss";
 @import "./views/settings/_EmailAddresses.scss";
 @import "./views/settings/_IntegrationManager.scss";