and use it for retreiving the status of an author faster.
Also add tests for both.

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.

Comments and reblogs need to depend on the post they refer to.
Since message dependencies are limited to one group,
the post and also the comments need to be wrapped
when commented on or reblogged to another blog.

For this reason, in addition to comments, two new wrapping message types
are introduced. They retain all data of the original messages and allow
for reconstruction and signature verification.

This commit breaks backwards compatibility with old blog posts.
It removes the content type, title and parent ID from the post
message structure.

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.