Add a confirmation prompt before actually enabling encryption

[turt2live]

[TUSF]

The prompt should also make obvious that, not only will the change be permanent, but what "Encryption" actually means for a room.

[uhoreg]

Warning could be something along the lines of: "Encryption may prevent integrations such as bots and bridges from working correctly in this room. Once encryption is enabled, it cannot be disabled."

[turt2live]

Fixed by matrix-org/matrix-react-sdk#2728