Make cross-signing dialog clearer and more context-aware

- Don't show loading spinners while waiting for user action
- When checking if there are other devices we can verify against, only
  look for devices that are actually cross-signed.
- Adjust displayed options depending on whether other devices and/or
  recovery keys exist, and add an option to reset cross-signing keys
  if necessary.
- Various minor clarifying adjustments to UI styling/text

Signed-off-by: Faye Duxovni <fayed@element.io>

res/css/_components.scss

@@ -38,6 +38,7 @@
 @import "./structures/_ViewSource.scss";
 @import "./structures/auth/_CompleteSecurity.scss";
 @import "./structures/auth/_Login.scss";
+@import "./structures/auth/_SetupEncryptionBody.scss";
 @import "./views/audio_messages/_AudioPlayer.scss";
 @import "./views/audio_messages/_PlayPauseButton.scss";
 @import "./views/audio_messages/_PlaybackContainer.scss";