Unify all private message responses in one PrivateResponse class

...g/briarproject/briar/android/blog/BlogControllerImpl.java

...iarproject/briar/android/contact/ContactListFragment.java

...arproject/briar/android/contact/ConversationActivity.java

.../briarproject/briar/android/contact/ConversationItem.java

...briarproject/briar/android/forum/ForumControllerImpl.java

...ndroid/privategroup/conversation/GroupControllerImpl.java

...g/briarproject/briar/api/blog/BlogInvitationResponse.java

...r/api/blog/event/BlogInvitationResponseReceivedEvent.java

...briarproject/briar/api/forum/ForumInvitationResponse.java

...rproject/briar/api/introduction/IntroductionResponse.java

...org/briarproject/briar/api/messaging/PrivateResponse.java

...vategroup/event/GroupInvitationResponseReceivedEvent.java

.../api/privategroup/invitation/GroupInvitationResponse.java

...ar/api/sharing/event/InvitationResponseReceivedEvent.java

...iarproject/briar/introduction/AbstractProtocolEngine.java

...arproject/briar/introduction/IntroductionManagerImpl.java

.../briar/privategroup/invitation/CreatorProtocolEngine.java

...r/privategroup/invitation/GroupInvitationManagerImpl.java

...briarproject/briar/sharing/BlogInvitationFactoryImpl.java

...org/briarproject/briar/sharing/BlogMessageParserImpl.java