Change to nag bar for incoming verification

[dbkr]

Show a nag bar at the top of the screen as per designs on an incoming verification request rather than popping up a dialog straight away.

• At this stage we can still show the same dialog when you click on the bar

• create a new toast component, flexible enough to show any kind of notification
• add verification request toasts for crypto.verification.start/Room.timeline verification events on client, open sas dialog when accepting

Remaining things

• when accepting or cancelling request through timeline tile, make toast disappear
• accepted or declined requests still show on load
• undefined name in toast
• switch to DM with accepting request on toast
• feature flag the toasts
• make it work for legacy to_device verification requests as well
• merge FormButton styles with buttons used in key verification tile