Certified Transcript for pre-join history messages

[BillCarsonFr]

Follow-up of #749.

As per MSC3061 , on rooms that support it (m.room.history_visibility), it is possible for a joined user to access history of the room (pre-join).

Currently such messages will be marked with a gray shield warning (The authenticity of this encrypted message cannot be guaranted on this session)

We want to avoid these warnings to avoid user fatigue (and it's mostly noise), and instead use the notion of certified transcript, i.e. History as shared by <xx>

All SDK has now a way to detect and handle such keys shared on invite. UX need to be updated to not show warning (gray shields), but instead:

This message should be shown as a top info banner dismissible, every individual message shouldn't be marked with a warning. This info could be accessed via some menu if needed (cryptographie info of message).