A fake ACTIVATE message would cause us to activate the transport keys
before the contact has received our auth message,
which would compromise forward secrecy.

Previously, the introducer would process and forward invalid messages by
the introducees. This commit adds the necessary checks and tests.

When the user attempts an introduction, instead of the introduction
message input field, an explanatory text will be shown and the
introduction can not be made until the last one has been finished.

Closes #308

Add constant-time method for verifying MACs

See merge request akwizgran/briar!773

Make test data creation configurable.

Closes #1149

See merge request akwizgran/briar!771

Create a valid BT MAC and UUID when creating testdata

See merge request akwizgran/briar!769

Helper methods for dealing with TransportProperties

See merge request akwizgran/briar!768

Add download button to ExpiredActivity

See merge request akwizgran/briar!766

Helper method for parsing transport properties

See merge request akwizgran/briar!764

Generate message and group IDs in a forward-compatible way

See merge request akwizgran/briar!763

Don't download Tor binaries if they already exist

See merge request akwizgran/briar!760

Set max length for client IDs

See merge request akwizgran/briar!762