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.

The MAC and signature are not yet generated and verified.
This will happen in a later commit.

This also fixes a bug where new feeds was not added properly.

Also fixes one bug where a new feed was not saved and
improved HTML stripping a bit.

This allows simpler server-side code, and a failure part-way through sending won't require restarting from the beginning next time.

Also, don't support unverifying contacts.

Closes #580

Also includes unit tests for the new message types.

Closes #591

Only personal blogs from non-contacts can be removed.
This also adds integration tests that check if blogs can actually be removed.

Closes #579

Fixes #470

Briar crashed when run in a device without bluetooth or without camera
such as an emulator.

Closes #514

This commit refactors the code for sharing forums,
so it can be used for sharing blogs as well.

It does not yet include code for responding to blog invitations.

Closes #486

This is done by remembering the time of the latest entry.
All entries newer than that are considered new and will be posted.

Closes #485

* Implemented in briar-core as a `ScheduledExecutorService`
  that gets started when the app starts
* The briar-api has a `FeedManager` interface
  that the UI can use to register and unregister feeds
* In this first iteration, feeds are fetched via HTTP(S), not Tor

Closes #484

The old behaviour was a leftover from the days of limited retention periods. The new behaviour will interact better with dependencies and message queues.

Closes #472

Database queries for metadata only returned it for messages that were delivered already.
However, there are cases (e.g. a pending message needs to be delivered) where
the validator needs to retrieve the metadata from the database.

For these cases, a special database query has been introduced.

The forum UI depended on sync layer events such as MessageStateChangedEvent.
Now, the forum client broadcasts its own high-level event (`ForumPostReceivedEvent`)
with the information the UI needs (`ForumPostHeader`).

Closes #310

This prevents trying to access the same group metadata in different groups.
Also, the conversation does not need to be reloaded once introduction messages arrive.

Closes #535

This commit addes a combined blog feed that shows all posts of all
subscribed blogs in the order the blog posts have been received.

For now, this commit also hides other blog functionality like adding
additional blogs and browsing individual blogs.

Closes #417

Closes #326

ContactExchangeTask was originally extracted from the Bluetooth invitation
task; this completes the refactoring.

This commit makes sure that there is only one identity manager that is
initialized early enough so hooks can be added properly.

Closes #446

This is useful when sorting blog posts in the combined feed. It ensures
you do not miss older posts that were just synced to you.