Before the introducee sends her ACK,
she derives a master key from the ephemeral shared secret as before.
Two nonces and a MAC key are then derived from the master key.
The local introducee signs one of the nonces and calculates a MAC
over her own identity public key, ephemeral public key,
transport properties and timestamp.
The local introducee includes the signature and MAC in her ACK.
On receiving the remote introducee's ACK,
the local introducee verifies the signature and MAC.
Should the verification fail, an ABORT is sent to the introducer and
the remote introducee that was added as inactive is deleted again.
Methods for creating, adding and removing forums have been moved to the
`ForumManager`. In order to still handle removing forums properly, a
`RemoveForumHook` has been introduced.
Methods for sharing forums with all current and future contacts have
been removed along with the localGroup where this information was saved.
The `ShareForumActivity` now has the proper label.
The `SessionId` and the `ProtocolEngine` have been moved to the
`clients` package.
This addresses part of #322 and part of what has been discussed in #320.
* force decline when two of our own identities are introduced to each
other
* throw away introduction requests to the same identity
(impossible to trigger from UI)
Closes#284
