Inform users of urbit-side key propagation time

[kennyrowe]

When a new Urbit ID creates its network keys it can take up to a half-hour for those keys to propagate to the network.

This fact is not messaged in bridge and often results in user confusion.

Please add a user-facing message to let the users know that it will take some amount of time for their Urbit to work.

[liam-fitzgerald]

cc: @jyng