Merge branch '2162-mailbox-pairing-ui' into 'master'

Implement UI for pairing Briar with mailbox

See merge request briar/briar!1585

bramble-core/src/test/java/org/briarproject/bramble/test/TestFeatureFlagModule.java

@@ -25,6 +25,11 @@ public class TestFeatureFlagModule {
 				return true;
 			}
 
+			@Override
+			public boolean shouldEnableMailbox() {
+				return true;
+			}
+
 			@Override
 			public boolean shouldEnablePrivateGroupsInCore() {
 				return true;