Merge branch '204-support-panic-kit' into 'master'

Initial Support For PanicKit

This MR adds support for the PanicKit libary. It supports the basic default panic response (signing out) which does not require user interaction to work. For destructive panic responses, the user needs to "connect" the panic trigger app (e.g. Ripple) with Briar first. To secure this connection further, TrustedIntents have been added. Gradle Witness is being used to guarantee the integrity of external libraries.

Otherwise, this MR is pretty isolated and should not interfere with other parts of Briar. So merging it should be safe. However, the panic functionality still has rough edges and should not be relied on just now. It will mature along the the PanicKit library and the Ripple app.

Closes #204 


See merge request !52