Switch to verifying users rather than devices

[dbkr]

This probably ought to be coupled tightly to #11212 since between doing that and this, we'll be in a weird quasi-state where we've set up cross-signing but new verifications will still be verifying devices.

In the MemberInfoPanel, if the cross-signing labs flag is enabled, hide verify buttons on the individual devices (moved to #11668) and instead show just one for the user. This button does a cross-signing verification. Use existing verification UI for now, placeholder UI for the verify button at this stage.

[jryans]

matrix-org/matrix-react-sdk#3728 is enough to verify users, but it does not currently hide the devices as well. From chat in the room, we may want a separate issue to track hiding the device list in case it's more complex than just hiding?

[jryans]

Or does "hide verify buttons on the individual devices" mean the device list would stay, but just the verify buttons per device removed...? Either way, let's track with a separate issue: #11668