History sharing: UX for telling users that history may be shared

[richvdh]

Algorithm:

• Whenever the user opens an encrypted room with history_visibility != joined, show the user a warning banner, unless the flag showing it has been dismissed is set. If the user dismisses the banner, set a flag in the client's local db recording that it has been dismissed for that room.
• Whenever the user opens an encrypted room with history_visibility == joined, clear the dismissed flag, if it is set. This ensures that if a room is changed public -> private -> public, we show the banner again when it is set back to public.

Clients:

• Web
• Android
• iOS

[richvdh]

Designs can be found at the figma links on #2829