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.

Show system notification for successful introductions

Includes code from https://github.com/consp1racy/android-support-preference
License: Apache License v2.0

Since API 23 this is required for access to the Bluetooth APIs
See:
https://developer.android.com/intl/ko/about/versions/marshmallow/android-6.0-changes.html#behavior-hardware-id

Closes #223

PanicKit does distinguish between two kinds of panic responses:

* default responses such as logging out which are non-destructive and
  do not require user interaction, so that the basics work without
  configuration
* destructive responses such as deleting user data. These require
  some sort of authentication to make sure they are not triggered
  by malicious apps

The second type of responses is implemented with this commit.

Authentication is done by comparing the package name
which is very weak. It requires the user to opt-in to
destructive responses and to configure from which app
to receive those (since there might be many different panic
trigger apps).

While possible to uninstall an app and install one with the same
package name afterwards, this always triggers notifications to
the user (if the attacker does not have root access).

Still that is no sufficient security for Briar's requirements,
so that TrustedIntents are used as well to make sure that the
app sending the destructive trigger is signed by a signing key
that we specified before. Currently, that is the one from the
GuardianProject and from IilabEngineering who does the Amnesty
International Panic App.

The responsibility of checking that the panic TRIGGER is
legitimate lies with the app responding to the trigger, so Briar
in this case. This commit checks whether the TRIGGER comes from
a trusted app before performing destructive actions,
but does perform the default action even when triggered from
untrusted apps.

Closes #210

This closes #204

The text of the startup failure notification is unhelpful due to lack of
space. Touching the notification now launches an activity that gives details
of the problem and what can be done about it.

Closes #38

This breaks backward compatibility for the wire protocol and messages
stored in the database. The database schema version has been
incremented.

Android L requires executables to be compiled with PIE support, so we'll
have to ship two Tor binaries.