plugin XEP-0384: OMEMO trust policy: OMEMO trust policy can now be specified. For now there are 2 policies: - `manual`: each new device fingerprint must be explicitly trusted or not before the device can be used, and the message sent - `BTBV` (Blind Trust Before Verification): each new device fingerprint is automically trusted, until user manually trust or not a device, in which case the behaviour becomes the same as for `manual` for the entity. When using the Trust UI, user can put the entity back to blind trust if they wish. A message is send as feedback to user when a new device is/must be trusted, trying to explain clearly what's happening to the user. Devices which have been automically trusted are marked, so user can know which ones may cause security issue.