Implement the new screens for the post-login security set up flow (which sets up cross signing and key backup).
We can't just directly stick this behind the labs flag since the idea is it pops up on login so you'd have to set the labs flag in your client config. Instead, let's:
- If the cross-signing labs flag is on and the user does not have cross-signing keys on their account, add a 'Experimental: setup cross-signing' button in the key backup section to start the initial setup flow. This is just temporary UI for development.
- If the cross-signing labs flag is on and the user does have cross-signing keys on their account, just show 'cross-signing enabled' text or something. Again this is temporary.
- On login, start the flow if the user has cross-signing keys on their account, but only support setup by entering recovery keys at this stage.
- This probably may as well be completely separate screens from the key backup screens: they're fairly substantially different, and this lets us keep everything as-is with the labs flag disabled.
Implement the new screens for the post-login security set up flow (which sets up cross signing and key backup).
We can't just directly stick this behind the labs flag since the idea is it pops up on login so you'd have to set the labs flag in your client config. Instead, let's: